AN: just a little idea I had. Kind of dark - this story has mentions of suicide, mentions of temporary major character death, and the main theme is mental illness (specifically disassociative identity disorder, more commonly known as multiple personality disorder). Spoilers for season 9.
When Jess dies, Sam breaks. Specifically, he breaks into two pieces.
He's always felt it - the darkness within him, thick and oozing from the back of his mind to the front, smothering all normal thought like thick, noxious fog. It's always been there.
But after she dies, it becomes too much - after his life on the road with his father and brother, forced to maim and kill and hunt (for better or for worse), he can't take it anymore. Jess used to see what he was - she didn't deny it like Dad, or try and cure it, like Dean. She dealt with it, she talked him down. She loved him anyway.
And now, she was gone.
So it gave itself a name. The darkness formed a whole other sense of self. It called itself Lucifer.
Dean almost laughs when he hears it. It sounds so melodramatic - especially because he's always thought of Sam's mental illness as something that they could get over; not something that could destroy them, and end the world.
Well, he always knew Sam was a little strange in the head - he would say the strangest things sometimes, and hear voices that they couldn't afford to battle with drugs - but he seemed to have gotten over it.
But one night several months after hiding what Jess' death had done to him, when he's particularly riled up, Sam lets go. Dean meets Lucifer. And he doesn't like him.
Sam's problems don't stop him from being a target for demons: on the contrary, they tell Sam to embrace the other person in his head. Sam is kind, and in control, and full of love and hope - Lucifer is none of these things. They know Sam will never lead them - but Lucifer will. If Sam lets Lucifer take control, then they'll win. If he uncages that beast, he'll never shut him away; never force him to the back of his mind, ever again.
Sam did end up letting Lucifer free; he did cause a lot of damage. But he regained control. He locked him away again. Permanently.
Four years after Sam's suicide, and subsequent resurrection, Sam starts acting strange again (in addition to the depression and the night-terrors, that is). He cocks his head to one side. He stares at Dean coldly, calculatingly - he'll start to say one thing in the relatively carefree, happy voice that Dean knows for sure is one hundred percent Sam, and then he'll pause and start a completely different sentence in a completely different tone of voice.
Dean doesn't want to ask. But he knows he has to - if Sam's suffering from the same problems again, he has to do something about it - though he has no fucking idea what. The only thing that stopped it last time was Sam killing himself, and being resurrected; his pain being taken on by an angel.
That wouldn't happen again.
"Lucifer?" Dean asks cautiously, his stomach feeling queasy and his heart in his throat, when his brother has one of his turns and Dean realises Sam isn't home at that moment. Sam's eyes turn to look at him, curious yet calm.
"No," He replies, his voice low and soft, though still undoubtedly powerful, "My name is Ezekiel,"
"… Ezekiel," Dean repeats. This isn't happening. Please, no - not again. "Why didn't you - uh … Why didn't Sam tell me about you?" Dean asked cautiously, his voice coming out more choked than he'd have liked in front of this person, this stranger - he didn't seem as bad as Lucifer, but Dean knew as well as anyone that these other personalities could change quick as a flash; he was rolling the dice even talking to it about Sam.
But this had been happening for a few months, now. Since Sam's attempt at the trials - Dean wondered if Sam's mind had fractured again, to cope with his failure. He imagined the lure of the new guy's voice, in Sam's head, soporific, lulling him to sleep so it could take over - it's okay, Sam - I'll watch out for you. You'll never hurt again. Never fail anymore.
"Sam does not know about me," Ezekiel replied, looking down at Sam's body analytically, smoothing down his clothes. Dean felt ill when he realised that even Sam's hair was neater than usual, presumably due to Ezekiel's influence.
"Can you … Do you have to-" Dean's voice broke slightly, and he gathered himself, rubbing his eyes before summoning the strength to continue. "Do you have to do this, again?" He asked, pleading with Sam to listen to him. He knew he would never get through to him, though - never mind how apparently harmless this new personality was. Lucifer had started off civil enough, after all.
But he had to try.
Ezekiel gave a great sigh, and cast a pitying look in Dean's direction. "I am sorry, Dean," He said, his formal speech too much for Dean to handle - hammering home how Sam had retreated into his own mind - along with his answer: "But Sam cannot survive without me,"
That's the beginning of the end. Sam hadn't survived the last time this had happened. Dean is almost certain that his brother won't survive this, either - his fears only intensify when the new personality starts to call itself Gadreel; starts refusing Sam his agency; killing innocents, and calling it 'God's will'.
But Dean couldn't do anything to expel Gadreel. He wasn't a demon, or a spirit, or an angel. He was part of Sam. If Gadreel died, Sam would die.
If that happened - or when it happened - Dean knew it would be the end for him, too.