With sinister flame and brimstone, the caster completely consumes one dead body so that absolutely nothing remains.
A creature whose body is destroyed by a charnel fire spell can only be brought back to life through a true resurrection spell.
If this spell is cast upon a corporeal undead, the creature gets a Will saving throw, and the caster must overcome its spell resistance (if any).
If the undead creature fails its save, it is destroyed forever.