Ghostly Fireball
Ghostly Fireball is a monster-only spell which behaves much like the Fireball spell, dealing heavy, unavoidable damage to everything caught in its 3×3 blast radius. Unlike Fireball, however, it deals partially-irresistible negative energy damage, ignores half of its victim's AC, and heals any undead affected by it.
The following enemy casts Ghostly Fireball: