Hades is living peacefully with his new wife in the human world when gruesome deaths committed in the past began happening again in the present.
And in every scene where the deaths occur, Hades's blood is left behind.
Azael and Iris, Hades's former guardians lead the investigations of the deaths.
Because of the result of the investigation, Hades found himself trapped in the world of Athenaeum, the world of the grey souls.
If Hades tries to leave Athenaeum, all his memories would be wiped away and he would no longer remember who and what he is.
He would no longer be Hades but someone else.