This is a magic item from the Warhammer 1st Edition adventure Fear the Worst, which I converted to 3rd Edition recently, and ran for my group as a side adventure over two sessions. After they killed the mutants in the castle the PCs discovered that the mutant leader, a wizard of Tzeentch (the chaos god of mutation and change) had a magical prism in his possession, which he appeared to be doing some kind of research on. They couldn’t identify it at the time, but after they took it back to Ubersreik and paid a mate, they learnt its powers – or so they thought.
Pedro’s Corrupting Prism
This magical prism is an unprepossessing, ordinary-looking glass prism, but if viewed with magical sight clearly holds a strong, neutral magic aura. Ordinary bright light (e.g. daylight) shone through it will split into the 7 colours of the rainbow just as it would through any normal, properly-used prism; but if a (mostly) pure member of the Bright or Celestial Orders shines a magical light through it, a much more sinister effect obtains. The light will split into bizarre, sickening patterns that give off a disturbing, dark magic aura. This is the only hint that any magic user can obtain that the prism is, in fact, powerfully cursed in the service of Tzeentch. The prism confers a powerful boon on its user, but comes with a secret and ultimately deadly curse.
Enhanced Energy Summoning: When carried, the prism enables a wizard of any order to use the Channel Energy action as a free action without any penalty, once per round. This is an enormously seductive power, since it enables the wizard to draw energy and cast a spell every round, without suffering the usual penalties on the use of the spell that obtain from the usual quick-casting process.
Corruption Curse: Every time a Wizard who is not a servant of Tzeentch uses this prism, have the player roll a dice pool consisting entirely of a number of challenge dice equal to the number of times the prism has been used in that encounter. That is, the first time the player uses the prism in an encounter, he or she rolls one challenge die; the second time, 2 challenge dice; and so on. If any chaos stars appear in the results, the player must immediately make an easy (1 challenge die) Resilience check. If this check fails, the player must immediately draw a single mutation from the mutation card deck.
Optional extra evil: If the GM wishes, he or she may roll the challenge dice secretly, to make it more difficult for the player to identify the cause of the mutations. Because there is a 12.5% chance of a chaos star on a single challenge die, this optional way of resolving the curse is likely to lead to the destruction of the PC, as they will incur multiple mutations before they realize the cause, and is only recommended for GMs who themselves serve the Lord of Change.