Acolyte of the Skin

From DevroWiki

There exist evil tomes and rituals held by a select cabal of evil renegade spellcasters that allow an initiate to bind the power and will of a fiend to their very soul, altering forever their mind and body. These rites are anathema before the Tribunal, and those they discover with knowledge of the practice of these rites are usually turned over to the Magistratum.

Even these deterrents - expulsion from the magocracy, shunning by one's peers, and being forbidden from ever practicing magic without fear of arrest or worse - even these are not enough to stop some from seeking this forbidden power, and there are those who will teach it, for a price.

This class is not considered open gaming content. It can be found in Complete Arcane, pg. 19, by Wizards of the Coast.