The character has learned at least 30 levels of formulaic spells from a source that has been corrupted in some way. Use of a corrupted spell for a selfish or sinful action receives a +3 bonus to the character’s Casting Total and Penetration Total. Succeeding in a roll because of this bonus (that is, when without it the spell would have failed or cost Fatigue) means he immediately acquires 5 experience points towards mastery of that spell. However, uses of the spell that are neutral or selfless receive a –3 penalty, and if you fail the roll or he fatigues himself because of this penalty, he immediately loses 5 experience points. If this would result in negative experience, he forgets the spell completely.
Any use of a corrupted spell is tainted and appears unholy. You may only take this Flaw once, though it can affect as many of the character’s spells as you wish.