Board Thread:Lore Discussion/@comment-95.103.57.198-20150111121424/@comment-1592115-20150305181022

They're either contained inside the body already or being brought back to the body with the spell, upon their death they'll say "Free, again." And the corpse will fade away into Ash Pile (or stays intact if done by Dead Thrall or The Ritual Stone)

Oddly, they still behave the same even when they're soul trapped before, it could be possibly it's either another soul, or they summoned the soul right from the Soul Cairn?