User blog comment:Serana Dawnsinger/Can undeath be cured?/@comment-36988138-20190718040003

Apparently the necromantic magics involved in undeath create buffer to prevent the soul from being joined with the body, which probably lingers well after the undead has been killed.

The only solution to undeath is really just a final death. The best that even a Naaru could do was to take a victim's soul to a better place.