On the season premiere of Supernatural…
Picking up where the season six finale left off, Castiel has become all powerful in consuming the souls of Purgatory.
He lets Sam, Dean, and Bobby live, but begins a widespread purging of angels who challenge him, and the humans he deems sinners.
Castiel also chooses not to kill Crowley, but rather, reinstate him to status as the King of Hell.
The longer he acts as the new "God," Castiel hears voices and his human vessel starts to deteriorate.
Meanwhile, Sam starts having hallucinations of hell that continue to get worse. He finally sees Lucifer who tells Sam that he is really in the cage. Sam has trouble recognizing what's real or not.
In order to stop Castiel, the brothers team up with Crowley to enslave Death and order him to kill Castiel. The spell works, but Castiel breaks the bond on Death.
Death reveals that in addition to souls, Castiel has taken in an ancient creature called the Leviathan, which is trying to get out. He tells the brothers that Castiel needs to send the souls and the Leviathan back to Purgatory.
The plan partly works, and Castiel returns to his old self. Unfortunately, the Leviathan never left him and it takes over Castiel.