Corrupt Spell( Book of Vile Darkness, p. 47)
The character can transform one of her spells into a thing of evil due to a deal she makes with an evil power.
Any evil alignment,
This feat adds the evil descriptor to a spell. Furthermore, if the spell deals damage, half of the damage is unholy damage. For example, a corrupted fireball cast by a 6th-level wizard deals 6d6 points of damage: 3d6 points of fire damage and 3d6 points of unholy damage. Thus, creatures immune to fire still potentially take 3d6 points of damage. The corrupted spell uses up a spell slot one level higher than the spell's actual level.
A character may take this feat multiple times, choosing a different spell each time.