So… this idea's been in my head for a while. And come on, who didn't think/hope Levi!Cass did this.

"Cass?" Dean asked worriedly, staring at the seemingly normal Castiel, before Castiel gave a wide grin.

"Cass, he's, um, he's gone. He's dead." Dean's eyes widened, the not-quite-Castiel's grin widening, "We run the show now." The next thing that happened startled Dean more than anything ever had.

The angel grabbed his shoulders, pulled him towards him and kissed him roughly on the mouth.

Dean's eyes widened in horror, staring at the fake Castiel, before he pulled away and threw Dean across the room, the Winchester sprawling out over the floor.

"Dude, what the hell?" Dean panted, half through pain, a bit through surprise, and a tad… (he never wanted to admit this to himself) pleasure.

"I am fulfilling the wishes of the angel we are possessing." The Leviathan controlled Castiel replied.

"The wishes? What are you…?" At that moment, the mouth of the Leviathan!Castiel opened, it's eyes glowing red and it's mouth smoking, as if trying desperately to scream, before it subsided and Castiel fell to the ground, unconscious.

"I think Castiel had a crush on ya boy." Bobby stated.

"Not a good time, Bobby!" Dean nearly yelled.

-SPN-

Castiel awoke with a start, his first coherent thought-

I'm tied up to a bed. It's a fairly large bed.

"Give us back Castiel, you sons of bitches." Dean demanded immediately, noticing he was awake.

"Dean, there is no need to-…" Castiel began.

"I repeat, give him back." Dean furiously grinded his teeth together.

"Dean, it's me." The belts proceeded to unloop of their own accord (his grace's doing), the angel standing up. "I expelled the Leviathans. It was easier without the weight of controlling the souls on my shoulders."

"Cass?" Dean croaked, a hopeful smile fleeting across his features.

"The Leviathans have been destroyed, and are expelled from my system." Castiel confirmed. "And what I said before still stands. I will find a way to redeem myself. I will stay here, if you want me to, and help find Crowley so he may pay for what has happened. I will rebuild what has been broken. I will save Sam. I promise you, Dean."

"Okay, Cass." Dean's mouth twitched, and he started to walk away from the angel, before stopping abruptly and turning.

"Cass, the Leviathan-…"

"I know, Dean." Castiel replied calmly.

"Was what they said, about wis-…"

"It was." Castiel stated.

"Okay." Dean awkwardly began to turn around and walk away again.

"Class continues tomorrow." Dean turned open, eyes wide and lips parted, to see Castiel giving a sly and smug expression.