The description for Corrupt Spell in Kedward Bone's stat block is incorrect for 3.5. Complete Divine updates it. It is a metamagic feat that gives a spell the evil descriptor and makes half the damage it does (if any)unholy damage and adds one level to the level of the spell for casting. Thus, Bone's Fireballs should take 2 of his 4th level slots.
The updated version of Corrupt Spell sure makes his Malign Spell Focus a hell of a lot more useful, too.