I’ll Stay With You
Read on AO3.
Castiel had been on his way to the bunker’s kitchen when he heard it, a kind of shouting, short and clipped. Frightened. It sounded like Dean was having a nightmare.
Cas stopped outside of Dean’s bedroom, leaning into the door to listen, waiting for the shouting to stop. Cas held his breath. This one seemed worse than the others, going on for much longer. He cracked open the door to Dean’s room. Dean was fast asleep, thrashing around in his bed, shouting out in fear at whatever was plaguing his sleep, but this time was different. Dean seemed to be hitting himself in his sleep, his hands curled up into tight fists, hitting himself in the stomach, the chest, the head, over and over and over with every gasp, every shout.
“Dean!” Cas shouted, rushing into the room. He moved over to Dean’s bed, taking the man by the shoulder, trying to get him to wake up. “Dean,” Cas repeated. “Dean, wake up!”
But Dean wouldn’t wake, and the hitting wouldn’t stop. Cas stood next to Dean’s bed, watching his friend hurt himself, not sure what to do. He just knew he needed to stop it.
Cas took off his trenchcoat and threw it over Dean’s chair. He slipped off his shoes and carefully climbed onto Dean’s bed. Cas brought Dean up against him, holding him tight against his chest, pinning his arms to his side. Dean continued to struggle, but after a few moments his breathing seemed to calm down, getting slower and slower.
When Dean woke up the next morning he found that he wasn’t alone. He noticed the presence of someone new before he opened his eyes, and for a while he lay there, breathing deeply, feeling safe. He didn’t know if Cas knew he was awake, but he snuggled into the crook of his arm a bit more, not wanting to ever move.
He felt tired, worn out; he knew he’d had a rough night. The sensation of waking up feeling like his mind had been punched over and over was not something new. But waking up in an angel’s arms, warm and protected - loved - that was new. And he never wanted it to go away.
“Thanks, Cas,” Dean murmured into the angel’s arms. Cas’s grip tightened, and Dean knew that Cas felt the same way.