Board Thread:False Info and Speculative Discussion/@comment-4772791-20160123002637/@comment-26879545-20160217185123

I personally believe that Parrish the human was dead when the bomb exploded, but because the Hellhound put its spirit inside of Parrish, it put him in a state similar to Kira; two spirits, (the human spirit and the supernatural spirit) fighting for control of the same body. The only difference here is that Kira was (presumably) born with hers while Parrish only got his because he was, for lack of a better term, at death's door the moment the Nemeton's power was restored.

As for what the Argents wanted with it, I think they wanted to resolve the conflict between the two sides so that the Hellhound could evolve and do its job. Human Parrish was afraid of losing control and becoming a danger to everyone he cared about, while Cerberus saw Parrish as a means to an end and didn't care whether the body died (again) after he killed the Beast. I think that's what Lydia was talking about when she was like "we need Parrish, and so do you," to him.