Board Thread:Q & A/@comment-24749544-20170919140531/@comment-31481772-20170919154314

Thanatos admits that the borders between life and death aren't foolproof. Therefore, in theory, a dead soul could escape to the living world.

Someone such as Melinoe could provide a secret path. Or Thanatos could turn a blind eye. Or some Hecate-level sorcery could return a dead soul to the living world temporarily but indefinitely.