Board Thread:Lore Discussion/@comment-95.103.57.198-20150111121424/@comment-121511-20150305181621

Mhazard wrote: 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?

The souls of the Undead are already long gone before they're raised, and it is outside of the ability of Necromancers to pull souls back from the Dreamsleeve/Oblivion/etc. Observe Draugr: their souls rest in the afterlife, but they still retain their skill with arms, their ability to use the Thu'um, and some aspects of their personality: the Undead are inhabited by vestiges of their personality, not their souls. Echoes of who they once were, if you will.