Castiel listens in pensive silence, hanging on every word, his expression tight with worry but laser focused nonetheless. The more John describes it, the better Castiel feels, because it sounds like this thing has less to do with the Darkness that he knows of than what he'd previously thought - though that makes it no less concerning.

There are a hundred things, a thousand things that could be at the root of this. Opening the Devil's Gate in Wyoming, breaking open the Cage, all the shattered seals of the Apocalypse, opening the door to Purgatory not only once, but twice.. plainly speaking, so much shit has been happening recently that it's impossible to say how it's affected the world, what consequences it might have wrought on this plane of existence, or all those linked to it. Thinning the barriers between dimensions seems a likely enough result, what with how many holes have been punched through the realms recently, how many more demons have been walking the earth, and angels as well, who meddle in human affairs far less than demons ever do.

"And an angel asked this of you? To look into it. To try to discover the source of this.. disease."

It's unsettling, to be sure. Castiel is high on Heaven's most wanted list these days, he can't claim to know all about everything that's going on up there, but the place is a mess, a wreck, factions following their own orders, keeping to private agendas. Everything has gone from black and white to varying shades of grey, and he no longer trusts his own kind intrinsically.