Board Thread:Off Topic Discussion/@comment-16838895-20141018144838/@comment-16838895-20141021011855

Well he swallowed their soul. :l It made him more powerful and he basically became god. But his body couldnt handle it and he started going insane (he was always pretty cray cray in the first place at this point) and his skin started to burn and melt and stuff. He had sam and deans help opening a portal to Purgatory where he basically barfed them up. The souls of the leviathan though stayed inside him, then took over and exploded out of him.