for this image prompt
Sam saw Lucifer everywhere. He knew he was crazy. Dean and Bobby knew he was crazy. He'd never questioned it, never thought it could possibly get worse -- until the day he realized he'd never been crazy to begin with.
The thing about being an archangel's vessel is that once you say yes it's forever. It didn't matter that Lucifer was still in his Cage. He had an open line to Sam, and so he amused himself by making Sam see him.
Sam learned how to shut Lucifer out. It took him a long time, and more discipline than any human had any right to have, but he did it. He kept himself from seeing Lucifer -- first for a day, then for two days, and then for a whole week.
Lucifer wasn't too pleased about that.
Sam had been Devil-free for nearly three weeks when things started to go wrong.
He was in the bathroom, drying off after his shower. The fog from the hot water had coated the mirror in white. Sam ran his finger across the surface, drawing a straight line down the middle. He wiped his hand across the glass in broad strokes, until he could see himself.
Then he spoke, but it wasn't his voice.
"I'm done playing nice, Sam."
Sam watched his lips move, he saw himself speak, but it wasn't him.
"You really think you can just shut me out? For good?"
Sam took a step away from the mirror and clasped his hand over his mouth. He didn't know what else to do. His legs shook so hard he had to lean against the wall to keep from falling to the floor.
It took him nearly five minutes to calm down enough to leave the room.
"What the Hell, Sam?" Dean yelled, the minute they were outside of the diner.
Sam couldn't think of anything to say. He also didn't want to risk opening his mouth again. Not after --
"How did you even know that? You scared the crap out of that girl, and then she ran away crying!" Dean stopped walking and spun on his heel, to face Sam. "What's going on with you, man?"
"Dean, I'm sorry, it wasn't --" Sam chewed on his lip, trying to figure out some way to explain it that didn't sound as horrible as it did in his head, "It wasn't me."
Dean blinked, "Come again? What do you mean it wasn't you? It wasn't you like...you're possessed?"
"No, no -- more like..." Sam pressed his thumb into his palm, rubbing at the old scar. "More like I just lost control for a minute there."
"Yeah you could say that." Dean turned around and kept walking across the parking lot. He didn't say another word the whole way back to their motel.
"Was it Lucifer?" Dean asked after they turned off the lights.
Sam swallowed, then answered, "Yes."
After a minute, Dean said, "I thought you said you didn't see him anymore."
"I'm not. It's been weeks, and then out of nowhere, the other day he just -- he spoke -- and there was nothing I could do about it. I thought maybe...I hoped it was a fluke."
"It's never a fluke Sammy, not with us."
Sam didn't trust himself anymore. He never had any warning that Lucifer was about to use him as a hand-puppet. It happened with Dean twice, and then it happened while they were on a case, while they were interviewing a victim's sister. Lucifer took over Sam's tongue just long enough to tell her exactly what Hell was like for those who'd committed suicide. Dean stammered an apology and dragged Sam out of the house as quickly as he could.
After that , Sam didn't trust himself on interviews anymore. He waited for Dean back at the hotel room, researching. He thought he was onto something -- everything they'd found was pointing to signs of a banshee.
When Dean came back an hour later, Sam told him what he'd found. That's what he meant to do anyway.
Instead of words, what came out of Sam's mouth was a horrible, piercing noise that brought Dean falling to his knees.
Dean covered his ears with his hands and huddled against the carpet.
Sam tried, he tried everything to make it stop. He slammed his hands over his mouth, but it didn't stop.
All around them, glass shattered. The window next to Dean flew apart and rained onto the carpet in large shards. Dean tried to look up at Sam, bracing himself with his left hand, and cut himself on one of the slivers there.
The mirror in the bathroom behind Sam exploded. Sam's eyes felt like they were burning and in the back of his mind, he could feel a horrible ache growing stronger with every passing second. It was the echo of Lucifer's power -- ancient, cold and infinite. Sam clenched his eyes shut from the pain, and because he didn't want to see anymore.
He closed his eyes, and the sound stopped. He heard himself laugh -- slow and deep -- and then he said, "That was me whispering. Want to hear me yell?"