The party had a simple task – escorting a wagon of mining goods from Neverwinter to the small town of Phandelver. An easy job through peaceful vales for our four adventurers, who were:

  • Mostly Simpson, A human Cleric of the Storm
  • Tyge Trip, a half-elf Paladin
  • Raymond d’Cantrus, a human wizard forced into adventuring after his grant funds dried up
  • Nithren Mar, “Mouse”, a half-elf rogue

They had been hired by a dwarf named Gundren Rockseeker, who dropped hints of a big find in the town of Phandelver and offered paltry pay for the group to escort his provisions in a slow wagon, while he hurried on ahead on horseback with his hired sword, Sildar Hallwinter. His fine speech gave the impression of riches and further jobs waiting to be had, so they agreed to his poor payment on the promise of future chances. Like good entrepreneurs, they hustled along the road to Phandelver, throwing caution to the wind in the hopes that they would move quickly and not burn their entire payment on provisions, and were soon within a half day’s ride of Phandelver, on the Triboar trail. It was here that the Goblins struck.

As they rounded a bend through a copse of trees they stumbled on two horses dead in the road, obviously shot down. Tyge moved forward to investigate, and a hail of arrows fell on them from both sides of the road. Tyge was struck down immediately, and in haste Mostly Simpson cast a fog cloud over the ground to the right side of the road, from which half the arrows had come. d’Cantrus hopped down from the wagon, and Nithren slipped into the shadows of the woods just as a harsh voice yelled “Gank the mage!” in goblin, and a second fusillade of arrows hit Mostly Simpson, felling him immediately. d’Cantrus crouched by the wagon, listening to the muttering and cursing of goblins in the fog to the far side of the road, as Nithren tried to creep up on the archers. Another volley of arrows felled d’Cantrus, leaving only Nithren upright. He ambushed one of the goblins as two others emerged onto the road and started looting the bodies, one on Mostly Simpson and one on Tyge. Mostly Simpson lay in a dire state, blood pouring from deep wounds, and his final moments fast upon him. Nithren fled deeper into the woods, drawing a goblin after him, and disappeared into the underbrush to lay another ambush.

It all looked lost, at least for Mostly Simpson, his last lifesblood draining into the mud of the road, when Tyge recovered consciousness, to find the goblin rifling her pack. It had already tried taking her greatsword but, finding it too heavy, cast it down at her side in disgust. Much obliged, Tyge hauled it around and smashed the goblin to pasty muck, rolling over and staggering to her feet to find another panicked goblin staring at her over Mostly Simpson’s paling body. The goblin, seeing her ichor-slicked sword and enraged face, panicked and ran for the brush, dropping all it had stolen. Tyge staggered over to Mostly Simpson and lay on hands, a gentle blue light suffusing him and bringing him suddenly alert. Staring around madly, he grabbed his spear and rushed to d’Cantrus’s side. Meanwhile in the bushes Nithren Mar killed a goblin, and was fast on the heels of another when the fleeing goblin came screaming past. They both turned and fled, but not fast enough – d’Cantrus struck one down with a bolt of wrathful magic, and only one escaped.

They had survived an unexpected goblin ambush, though only just. They regathered at the wagons to rest and recuperate, while Mouse inspected the horses. These horses were the same horses Gundren Rockseeker and his guard had left Neverwinter on some days before, shot down perhaps a day ago and thoroughly looted. With no sign of Gundren or his guard, the group presumed they must have been captured by the goblins. They decided to hide their wagon on the trail and follow the goblins to their lair, either to find and rescue Gundren or to take back his belongings if, as they feared, the worst fate had befallen him.

The goblin trail was easy to follow, and the traps they laid on it pathetically easy to spot and avoid. The trail led to a low cliff face with a cave entrance, obviously regularly used by goblins. A stream seeped out of the entrance, running strangely low in its banks for the end of the storm season, and a brace of bushes near the entrance contained an obvious goblin watchpoint. Mouse sneaked up to the watchpoint and lay low near its entrance. Here he heard the goblin that had fled the ambush, bragging about how he had single-handedly ambushed a party of warriors and driven them back down the road with his cunnning. As the other two goblins in the hide oohed and aahed at the brave goblin’s story, Mouse rose up behind him and cut his throat, pulling him back from the hide and into the open while grunting “No he didn’t!” as he ran.

The goblins came charging out after Mouse and straight into the trap, cut down on the river’s edge by arrows and spells. One fell immediately but the other fled into the cave. Not wanting the alarm raised, Tyge and Mostly Simpson charged in after. They stumbled in the dark, and were just orienting themselves when a ragged, starving wolf emerged from a cave mouth to their right. Mostly Simpson set his spear and the wolf impaled itself on the spear, but soon two more came charging out. Grimly beset by the river’s edge in the dark, they beat the two wolves down and killed the last goblin too. Silence fell on the caves, and blood slicked away down the torpid stream.

Though sluggish and shallow, in the narrow confines of the cave interior the stream drowned all sounds from further in the cave. Confident they had not been heard further in, they explored the cave to their right. It was a filthy, stinking kennel, where the three wolves had been confined on chains and fed scraps. Holding in their vomit, the party searched through the filth and the shit for signs of Gundren rockseeker but found no human bones or remains. At the back of the cave they found a natural chimney that carved a narrow passage up to a higher cave, from where they could hear the faint sound of voices. They opted not to climb it for a surprise attack, and returned to the main passageway. This passageway was wide enough for the stream and a narrow walkway next to it, suitable for them to walk in single file. They devised an elaborate strategy for the humans to move forward using a light spell cast on a stone, and Mouse crept ahead to investigate the tunnel. After a short time the rest of them came forward to join him. The passageway curved ahead of them, and a passageway branched off to their left from the far side of the stream. Ahead of them a very unsafe wooden bridge balanced precariously over the stream, linking passageways that crossed the tunnel they were in. As they stood their discussing which way to go, they heard a roar ahead of them and suddenly a wall of water surged around the bend of the tunnel, rushing towards them under the bridge. They dived in separate directions, Mouse for the side tunnel, de Cantrus and Tyge back to the filthy wolf cave, while Mostly Simpson just stood his ground and weathered the storm. As Mouse ducked into the side tunnel he threw the stone with the enchanted light into the swell, and it bobbed away into the darkness. After the water had passed they rejoined in the side tunnel and climbed hastily up its scree-strewn slope to a higher tunnel, hoping that any pursuit would pass them. The constant chattering of the stream hid the sound of any pursuit, and they found themselves standing in a narrow tunnel that obviously must cut back to the bridge under which the water had raged.

Mouse stalked left, away from the direction of the bridge. He had found a nest of goblins, and they needed the mage. The wizard came forward to witness a scene of goblin Hygge: Four goblins crouched around a fireplace while a fifth turned a piece of pig carcass on a spit, swigging a mixture of alcohol, oil and pepper and spitting it over the mouldering pig flesh for flavour, while a boss goblin yelled down at them from on high in a torrent of abuse and foul-mouthed suggestions for the cooking pit. Thick smoke roiled around the room and out of a narrow gap in the ceiling, and the walls were smeared with old oil fumes and smoke. One of the goblins limped to a foul pile of straw in the corner and unleashed a lurid stream of piss that steamed in the half light and seemed to cascade forever into the dank straws, while the other snarled and recommended the cook add it to the carcass to improve the flavour.

deCantrus whispered the incantation for a Sleep spell and they attacked. The battle was short and brutal – three goblins fell asleep under deCantrus’s magic, two were cut down by arrows, and the last fell to Tgye’s sword with barely a whimper. The cave was cleared. Having slaughtered his friends, deCantrus cast Charm Person on the cook, and they woke him with the intention of telling him they had saved him from the beast that struck down his friends. Unfortunately as soon as he woke up, the goblin began a terrified bleating and prostrating, a sure sign that the spell had failed. Instead they threatened him, and with barely a raised voice they convinced him to tell them everything he knew:

  • This was the Cragmaw clan
  • Their main lair was in Cragmaw castle, somewhere northeast of here
  • The boss in this cave was Klorg, a bugbear of stupendous power
  • They had been ordered by their chief in Cragmaw Castle to raid wagons to find a dwarf called Gundren and deliver him to the castle
  • They had done just that, but were keeping his guard here for food or ransom
  • There were two water traps at the end of the tunnel, but they could go over the bridge and sneak up on the people in the trap room
  • His name was Yelmick

They cut a deal with Yelmick: They would let him live if he fled straight to Cragmaw Castle and set himself up as a cook there. Then when they invaded that castle, he would be ready to give them all the help they needed. If he agreed, they would give him treasure. If he betrayed them, they would deal with him just as they were about to deal with his friends.

They weren’t his friends, he assured them – he was just the cook. He just liked to cook. He didn’t know about any of the bad stuff and was just acting on orders. He would happily go to Cragmaw Castle for them and set up as a cook, no doubt he would soon become cook to the chief himself, since he was such a great cook.

Mouse recommended him to make contact with a traveling bard called Michelin who rates cooks, and then bade the goblin wretch be on his way. Yelmick was not even out of the room before behind him he heard the crunching, gurgling sound of his tribe mates being put to death in their sleep. Not doubting his new patrons’ power, he fled with nary a backward look.

Their grim task done, the PCs turned towards the bridge, and prepared to invest the inner depths of the hideout.

 

 

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Keep slaughter in your heart. A life without it is like a sunlit garden where the flowers are blooming.

– From The Epigrams of Warboss Wilde, Alvin Redmane

The party were four in number:

  • Aurak the Unborn, Lawful Evil Half-orc monk
  • Nemeia, Chaotic Good Tiefling Witch
  • Kaylee Sparklegem, Chaotic Neutral forest gnome rogue (NPC)
  • Ufgram Ironfist, Neutral Good Dwarven cleric of life (NPC)

The four served Mistress Servaine, a retired adventurer who runs a stable of mercenaries based near Baldur’s Gate. She had been asked by an old friend, Shirlwan Hukrien, to send a team of adventurers to her home to find her son and daughter, who had gone missing some two weeks ago while exploring an old ruin known as the Sunless Citadel. The party had been chosen for this mission, which was not seen as a particularly challenging one, as an introduction to their service for Mistress Servaine.

They set off immediately, trekking the two days to nearby Oakhurst, where the Hukrien family lived. Here they introduced themselves to Shirlwan Hukrien, and discovered that though she was once a powerful wizard, recently her powers had begun to wane and she was not herself able to use them to locate her children. This strange weakness was not her unique cross to bear, she revealed, but had been noticed by other powerful wizards in the land of Faerun. Shrugging off Aurak’s suggestion that they immediately take advantage of this strange circumstance by raiding the tower of the Red Wizards of Thay, the PCs asked Shirlwan for the loan of some horses, and set off to the Sunless Citadel.

The Citadel is an old tower that had fallen into a ravine during some ancient cataclysm. The locals said variously that the ravine had swallowed the tower whole as a punishment from the gods for the evil dragon cult that occupied it, or that its collapse had been the result of a horrible magical experiment gone wrong, or that the earth is made of great plates of stone that sometimes move like scales on the back of a sleeping dragon, and in their clashing lay to ruins even the greatest achievements of mortals; in truth no one knew the real reason, but all avoided the tower and its environs. The tower was perhaps five days’ travel on foot, the far side of the Plains of Ash, and easily visible from afar because one ruined tower of the fallen citadel still stood lonely watch on the plain overlooking the cursed ravine.

They found it easily, but their approach to the shattered tower was interrupted by a strange ambush. The land leading up to the tower was rough terrain of scattered bushes and small stunted trees, all dusted with a fine silver-grey layer of dust from the nearby Plains. In the distance, a little removed from the lonely tower, they could see the occasional smoke towers from remote farms, which hereabouts primarily grew apples and some wild fruits that need much sunlight and open space to thrive. Passing through an abandoned and overgrown orchard from one of these farms, the group were attacked by a strange group of four humanoid things, composed of dried up vegetation, tangled vines and tree limbs, all bound together in some hideous mockery of humanity and animated by some vicious spark of magic. The things lurched out of the overgrowth towards the PCs, who, ever on their guard, leapt to the attack. Aurak the Unborn let fly a boomerang, which missed it tendriled target and returned true to his grey-skinned grip; Kaylee fired an arrow into one with no seeming effect on its relentless advance; and Nemeia and Ulfgram the cleric surged forward to melee, where the strange creatures of vine and stick attempted to claw them from their horses with hardened hands of stick and thorn. The beasts, though disturbing in countenance, proved little match for the four, and were soon vanquished. A brief search of their broken remains revealed nothing of interest or use.

“There is,” Aurak the Unborn observed, quoting his icon Warboss Wilde, “only one thing worse than being attacked; and that is not being attacked.” With these wise words, they moved on.

A short ride took them to the tower, empty and broken, where it overlooked the ravine. Here they found a rope knotted tightly to a rock, and dangling down to a ledge and a set of stairs that began some 20′ below. Taking this as sure sign that they were on the trail of their targets, the party were discussing how best to descend when they were attacked from behind by a trio of giant rats. Their logistical discussion briefly disturbed, they slaughtered the rats contemptuously, and descended the rope. Nemeia fell halfway, but they were only descending 20′ and she landed in a lucky position, taking little damage. From the first ledge they found functioning stairs, and descended smoothly to the ravine floor.

At the base of the ravine they found the uppermost level of the citadel itself. It truly had sunken into the valley floor, and much of it was ruined, but the top of the largest central donjon rose above the ravine floor, a single door offering entry into the tower. To their north and south the broken ground swallowed up the outhouses of the tower, but here in the centre the building looked relatively solid and safe, so they pushed open the door.

From inside more giant rats emerged to attack them, but they beat them down with ease and pushed their way inside, finding a large room with doors to the north and southwest. Four goblins had been killed in here, with one still pinned to the far wall by the spear that had killed it. They guessed that the adventurers they sought had passed successfully through here, and though they thought there was little chance of finding anything valuable, searched the stinking, grimacing corpses anyway. They found nothing, but during the search Aurak the Unborn found a secret door in the south wall. After a moment of preparation, they bid him open it.

Pushing the lever that opened the door, Aurak barely avoided a poisoned needle that nearly stuck his hand. The door slid open, revealing a small room with arrow slits that would once have overlooked the inner courtyard of the citadel, before the scales of the earth had ground together and dragged it down into hell. Four skeletons, of archers who must have once defended this room, lay in an untidy pile in the corner. As the party entered to search them these bodies twitched and rose up, drawing rusty shortswords and preparing to attack. Battle was joined, with Aurak and Kaylee fighting in melee while Ironfist and Nemeia conjured eery ghostly fists to strike at the undead from outside the room. Again they prevailed, taking only minor damage, and soon the bones were quiescent again. They found nothing especially valuable here, so they moved onward, through the door in the northern face of the main room.

Warboss Wilde says “We are each our own devil, and we make this world our hell”

Here they found a corridor, wide enough for them to pass along two abreast, down which they walked cautiously. When they were near the end they found a door on the left-hand side of the tunnel, which they opened and entered. Here was a small 10’x10′ room, with a strange keg-like structure in one corner. Two rusted iron pipes protruded from the keg and curved around into the floor, to what purpose none of them could guess. With some effort they hauled open the top of the keg, and immediately two nasty little demon creatures sprang out of the keg and attacked them – mephits! A steam mephit and an ice mephit, part of some infernal machinery that must once have warmed the castle or powered some ingenious torture device. This fight was harder than the last, and they struggled to hit and subdue the vicious little elemental spirits. When the steam mephit died it let loose a hideous cry of rage and explosion of steam that burnt them all, leaving them stunned and hurt. Ironfist cast a healing spell on Aurak, they shut the door and rested for a half dozen hours, and then they proceeded along the corridor to the door at the end.

The door opened into a much larger square chamber, that held a huge burnt-out firepit and a large steel cage, its bars smashed and burst open on the side facing them. Doors led out of the room at several points, and from a huddle of rags on the far side of a large stone table they could hear snuffling and whimpering. Once they had assured themselves the room contained no threats Nemeia marched over to the bundle of filthy rags and tore it aside, revealing a forlorn and sniveling kobold, its little draconic nose wrinkled in that expression of conniving self-pity that is characteristic of the lowest of the evil humanoids. Aurak raised his axe to end the piteous thing, but Nemeia gestured for him to hold. She hauled the little wretch up, not ungently, and began speaking to it in Draconic, its native language.

Tieflings. Never trust them.

After a short conversation she revealed that the kobold was one of a tribe living in the citadel, that its name was Mebo, and that it had been charged with looking after a white dragon wyrmling[1], that had been trapped in the cage behind them. Some goblins who shared the citadel with the kobold tribe had raided the room and stolen the dragon, and the kobold tribe held Mebo responsible. He was not allowed back on pain of death, unless he was bringing the dragon with him. Nemeia had asked Mebo about the adventurers they were tracking, and he said he knew nothing of any adventurers, so it was Nemeia’s guess that the adventurers had been captured by the goblins, or were in some desperate situation in the area where the goblins lived. She suggested that Mebo could take them to the kobold chieftain, and they could negotiate with the chieftain for a reward in exchange for returning the dragon. This would mean that they could pass unmolested through to the goblin area, with Mebo as a guide, and make haste to the adventurers they sought.

The rest of the party agreed with Mebo’s plan, and he took them down some corridors into a long room lined with ancient, crumbling statues. They passed through the statues into an area thronged with kobolds and reeking of their strange metallic, earthy smell, where they found the kobold chieftain. She wore a mouldering wizards cloak, cut down to size, and lounged on a throne of wood and rotting upholstery that must have been here when the castle was hurled down here by the gods. Behind her stood a platform adorned with various pointless and stupid kobold trinkets – a lizard brain, a rusted dagger, the usual kind of tawdry junk these strange fallen dragon-dogs value – but in amongst it sat a large and impressive bronze key on a special hook. That key obviously opened a treasure room somewhere in this patchwork of collapsed masonry.

They negotiated. The chieftain agreed with their suggestion, and offered them a paltry reward in exchange for returning the dragon. She agreed to let them take Mebo with them as a guide. When pushed about the key, she shrugged, and refused to give it to them because it looked pretty as an ornament behind her throne. They pushed her, and she agreed to loan it to them if they could return the dragon. A loan was all they need. They bowed appropriately, took their leave, and dragged Mebo away towards a door out of the throne room.

To the goblins, and glory!

 


fn1: an annoying recent trend in D&D modules is that they put in baby dragons for 1st level characters to kill, so we can feel like we’ve fought a dragon, but it leaves a bad taste in my mouth because it’s not a real dragon and I want those things kept for when I can really earn the feeling of success that killing a dragon brings with it.

We’ve all been there: Your PC is up against a much weaker opponent, deploying your primary power or skill, but in the crucial moment the d20 roll comes up low for you or high for the opponent, and you once again find that your best power failed you when you were sure it would work. This happens all the time in D&D because the d20 has a flat distribution and that means that low rolls are just as likely as high ones. Although this means on average you might expect your best power to work, unless you are absolutely obliterating your opponent you can’t rely on the dice to turn up even in the ballpark of where you need them to be. This is also a problem in Cyberpunk (d10) and Warhammer 2nd Edition (d100). I have always found it really frustrating, because if use a peaked distribution we can be fairly confident that the dice will roll around about the middle of their distribution more often than the edge. I have complained about this many times, but I have never bothered to see how big a difference a peaked distribution would make to the flow of the game. So here I compare the easiest peaked distribution, 2d10, with 1d20 as a basic die structure for D&D. I have chosen 2d10 because the average roll is about the same as 1d20, and its most likely value is close to the basic DC values of D&D, which are abut 9-11.

Method

For this analysis I have conducted three basic calculations, on the assumption that a PC (the “attacker”) is in a challenged skill check with another PC or enemy (the “defender”):

  1. Comparing the probability of success for the attacker for every die roll on a 1d20 and a 2d10 basic roll
  2. Estimating the total probability of success for the attacker across a wide range of possible skill bonuses, and comparing these probabilities for 1d20 and 2d10
  3. Comparing the probability of success for a highly skilled attacker against a low-skilled attacker, across a wide range of defensive bonuses

For objective 1 I have performed the calculations for attackers with skill values of +0, +4 or +8, against a defender with a bonus of +4 or +0. The specific pairings are shown in the figures below. I chose +4 because it is the basic bonus you can expect for a 1st level character using their proficiency bonus and their best attribute, and +8 as a representative high bonus. For objective 2 I have calculated total probability of success for attackers with bonuses ranging from -2 to +10, against defenders with skill bonus of +0, +4 or +6. I chose +6 because this is the typical bonus you expect of a 5th level character who is working with their proficiency and has sunk their attribute bonus into their top attribute. For objective 3 I have compared a PC with a +6 bonus to a PC with a +0 bonus, for defense bonuses ranging from -2 to +10.

Probabilities of success for any particular die roll are easily calculated because the distributions of 1d20 and 2d10 are quite simple. Total probability of success is calculated using the law of total probability as follows:

P(success)=P(rolls a 1)*P(defender doesn’t beat 1)+P(rolls a 2)*P(defender doesn’t beat 2) +…

I have presented all results as graphs, but may refer to specific numbers where they matter. All figures can be expanded by clicking on them. Analyses were conducted in R, which is why some axis titles aren’t fully readable – you can make them bigger but then they fall off the edge of the graphics window. Stupid R!

Results

Figures 1-3 show the probability of success for every point on the die (from 2 to 20) for 1d20 vs. 2d10. In all figures the 2d10 is in red and the 1d20 in grey, and a grey vertical line has been placed where the probabilities of success are equal for the two die types.

Figure 1 shows that the 1d20 has a better chance of success for all die rolls between 2 and 15. That is, if you have a bonus of +0 and the defender has a bonus of +4, you are better off in a 1d20 system for almost all rolls. The point where the probabilities for 1d20 and 2d10 are equal is a die roll of 16. This corresponds with the defender needing a 12+, and all die rolls after this (17-20) correspond with the defender needing to get a high number on the downward peak of the 2d10 distribution. It may seem counter-intuitive that the 1d20 system rewards you for rolling low, but it is worth remembering that the comparatively low rolls – below 10 – are less likely on a 2d10, so although if you do roll one you are less likely to succeed than if you had a 1d20 system, you are also less likely to roll one. We will see how this pans out when we consider total probability of success, below.

Figure 1: Probability of success at die rolls from 2-20 for 1d20 and 2d10, where attacker has +0 bonus and defender +4

Figure 2 shows the probabilities of success for an attacker with +4 and a defender with +0. In this case we expect the attacker to win on a wider range of dice rolls, and this is exactly what we observe. Now the point where 2d10 is better for the attacker than 1d20 corresponds with dice rolls of 8 or more – in this case, dice rolls that the defender needs to get 12 or more to beat. We see the same process in action.

Figure 2: Probability of success at die rolls from 2-20 for 1d20 and 2d10, where attacker has +4 bonus and defender +0

Figure 3 shows the probabilities of success for an attacker with +8 and a defender with +0. Now we see that the 2d10 is more beneficial to the attacker than the 1d20 from rolls of 4 and above – again, the point beyond which the defender needs to roll 12 or more.

Figure 3: Probability of success at die rolls from 2-20 for 1d20 and 2d10, where attacker has +0 bonus and defender +4

These results are summarized for two cases in Figure 4, which gives the odds ratio for success with a 1d20 compared to 2d10 at each die roll. The odds ratio is the odds of success with a 1d20 divided by the odds of success with a 2d10, calculated at the given dice roll point. I use the odds ratio because it is the correct numerical method for comparing two probabilities, and reflects the special upper (1) and lower (0) bounds on probabilities. The odds ratio grows rapidly as a probability heads towards 0 or 1, and reflects the fact that a 10% difference in probability is a much more meaningful difference when one probability is 10% than when one probability is 50%.

 

Odds Ratios of success for 1d20 vs. 2d10, for two attacking cases

In this case I have shown only the case of an offense of +4 and a defense of +0, and an offense of +8 vs. a defense of +0. I used only these two cases because the case of +0 vs. +4 has such huge odds ratios that it is not possible to see the detail of the other two cases. This figure shows that for an offense of +4 and a defense of 0, the 1d20 has 2-3 times the odds of success at low numbers, but also much lower odds of success at high numbers. Effectively the 2d10 smooths out the probability patterns across the die roll, so that you get less chance of success if you roll poorly, and more chance of success if you roll well, compared to a 1d20.

Figures 5 to 7 show the total probability of success for 1d20 and 2d10 in three different cases. The total probability of success is the probability that you will beat your opponent when you roll the die. This is the probability you roll a 2 multiplied by the probability your opponent rolls greater than you, plus the probability you roll a 3 multiplied by the probability your opponent rolls greater than you, up to the probability you roll a 20. I have calculated this for a range of attack bonuses from -2 to +10, against three defense scenarios: 0, +4 and +6.

Figure 5 shows the total probability for 1d20 and 2d10 when rolled against a defense bonus of 0. Probabilities of success for both 2d10 and 1d20 are quite high, crossing 50% at about an attacking bonus of +0 as we would expect. The 2d10 roll has a lower probability of success than 1d20 for bonuses below 0, and a higher probability of successes for bonuses above 0.

Figure 5: Total probability of success against defense bonus of +0

Figure 6 shows the total probability of success for 2d10 and 1d20 against a defense bonus of +4. The ability of the 2d10 system to distinguish between people weaker than the defender and stronger than the defender is clearer here. At an attack bonus of -2 (vs. defense of +4) the 2d10 system has about a 10% lower chance of success than the 1d20; conversely, at attack bonus of +10 (vs. defense of +4) it has about a 10% higher probability of success. Both systems have an approximately 50% chance of success at a bonus of +4, as we expect.

Figure 6: Total probability of success against defense bonus of +4

Figure 7 shows the total probabilities against a defense bonus of +6. Again we see that the 2d10 system slightly punishes people with a lower bonus than the defender, and slightly rewards people with a higher bonus.

Figure 7: Total probability of success against defense bonus of +6

These results are summarized as odds ratios of success for 1d20 vs. 2d10 in Figure 8. Here the odds ratios are charted for the full range of attacker bonuses, with a separate curve for defense bonus of +0, +4 or +6. Here an odds ratio over 1 indicates that the 1d20 roll has a better chance of success than the 2d10, while an odds ratio below 1 indicates the 2d10 roll has a better chance of success. From this chart you can see that for all offense bonuses lower than the defense bonus, the 1d20 system gives a higher probability of success than the 2d10 system. As the defense bonus increases this relative benefit grows larger.

Figure 8: Odds Ratio of success for 1d20 vs. 2d10 across a wide range of offense bonuses, for three defense bonuses

 

The odds ratio curves in Figure 8 raise an interesting final point about the 2d10 system vs. the 1d20 system. Since the 1d20 system has higher probabilities of success at low offense bonuses, and relatively lower probabilities of success at higher offense bonuses, it should be the case that the difference in success probability between a skilled PC and an unskilled PC will be smaller for the 1d20 system than for the 2d10. That is, if your PC has a bonus of 6 and is attempting to do something, he or she will have a higher chance of success than a person with a bonus of 0, but the relative difference in success probability will not be so great; this difference will be more pronounced for someone using 2d10. To put concrete numbers on this, in the 1d20 system a PC with a +6 bonus trying to beat a defense of +2 has a 65% chance of success, while a PC with a +0 bonus has a 39% chance of success. In contrast, using 2d10 the PC with the +6 bonus has a 72% chance of success, while the PC with the +0 bonus has a 34% chance of success. These greater relative differences are important because they encourage party diversification – if people with large bonuses have commensurately better chances of success than people with small bonuses, then there is a good reason for having distinct roles in the party, and less risk that e.g. even though someone has specialized in stealth, the chances that the non-stealthy people can pull off the same moves will be high enough that the stealth PC does not stand out.

This effect is shown in Figure 9, where I plot the odds ratio of success for a PC with +6 bonus compared to +0 bonus, against defense bonuses ranging from -2 to +10, for both dice systems. It shows that across all defense bonuses the odds ratio of success for a PC with +6 bonus is about 3 times that for a person with +0 bonus when we roll 1d20. In contrast, with 2d10 this odds ratio is closer to 6, and appears to grow larger as the defense bonus increases. That is, as the targeted task becomes increasingly difficult, the 2d10 system rewards people who are specialized in that task compared to those who are not; and at all difficulties, the difference in success chance for the specialist is greater than for the non-specialist, compared to the 1d20 system.

Figure 9: Odds ratio of success for bonus of +6 vs. +0, in both dice systems, against a wide range of defense bonuses

 

Conclusion

Rolling 2d10 for skill checks and attacks in D&D 5th Edition makes very little overall difference to the probability distribution of outcomes, but it does slightly change the distribution in three key ways:

  • It increases the chance that a high dice roll will lead to success, and reduces the chance of success on a low dice roll;
  • It lowers the probability of success for PCs targeting enemies with higher bonuses than they have, and raises the probability of success for PCs with higher bonuses;
  • It increases the gap in success chance between specialist and non-specialist PCs, rewarding diversification of skills and character choices

The 2d10 system does not change the point at which the PC has a 50% chance of success, but it does reduce the probability of criticals. It is worth noting that with a 2d10 system, the process for advantage requires rolling 4d10 and picking the best 2 (rolling 3d10 and picking the best 2 actually reduces the probability of a critical hit). Some might find this annoying, though those of us who enjoy dice pool games will be happy to be rolling 4d10. For those who find it annoying, dropping advantage altogether and replacing it with +3 will likely give the same results (see e.g. here and here). But if you like rolling lots of dice 4d10 choose 2 sounds more fun than 2d20 choose 1.

I don’t think that switching to 2d10 will massively change the way the game runs or really hugely unbalance anything but it will ensure that when you roll high you can have high confidence of success against someone of about your own power; and it will ensure that if you are the person in the party who is good at a task (like picking locks, sneaking, or influencing people) you will be consistently much more likely to do it than the rest of your group, which is nice because it makes your shine really shine. So I recommend switching to 2d10 for all task resolution in D&D.

A final note on DCs

The basic DC for a spell or special power used by a PC in D&D 5e is 8+proficiency level+attribute. This means that against someone with proficiency in the given save and the same attribute bonus as you, they have a 60% chance of avoiding your power. I think that’s very poor design – it should be 10+proficiency+attribute, so that against someone with your own power level you have a 50% chance of success, not 40%. It could be argued that 40% is reasonable since people often take half damage on a save and the full effect of a spell is quite serious, but given wizards have few spells (and most other powers are restricted in use), this doesn’t seem reasonable. So I would consider adding 2 to all save DCs in the game, regardless of whether you switch to 2d10 or stay on 1d20.