Dead Thrall

Dead Thrall is a Master level Conjuration spell that permanently raises a dead body to fight along side you. Summoning another Atronach or raising an undead will cause the Dead Thrall to turn back into a lifeless body, but will not disintegrate so it can be reused as many times as possible. Reanimated bodies sometimes say "Finally.." when killed.

Spell Tome Locations
The spell can be purchased from Phinis Gestor after completing the quest Conjuration Ritual Spell, which requires at least 90 Conjuation.