While Azrael was on the way to the Netherworld, his mind was occupied by Selene.
Azrael had a nagging feeling about this. Lots of mysteries kept on happening around Selene.
She died accidentally even though it was not her time yet. Her name was not included in the book of death.
Sienna sent Eros to him to make a deal, helping Selene's soul to return to the human world and live again.
When Selene died that day, that was also the same day a soul escaped from the Netherworld.
The escaped soul came back to the human world, possessed a body, and killed someone. The escaped soul turned into a dark soul after killing someone.
The soul who was killed by the escaped soul was still missing and they couldn't trace it. But later on, they found out that the escaped soul might be related to Selene.
It came back for Selene. And now, it was targeting the people who tried to harm and hurt Selene including Micaela.