A quick ficlet I wrote based on this picture by BadBastion and you can find it on their Tumblr here: post/32575146013/wincestifer-or-samifercest-idk-but-i-like-it - leave them a nice comment!
Whenever Sam thought that Lucifer's imagination had run dry and that he had no new tricks and pranks to pull on him to make his already miserable life just that much more unbearable, it seemed as if the beast came up with new and ingenious ways to torture the young Winchester. Razorblades hidden in his food, the shadows that kept creeping up on him when he least expected it and the one time he had dared to eat and found that his sandwich had turned into a blood-covered fetus. His imagination seemingly had no limits.
But the worst thing that Lucifer ever put him through, something he would do on occasion when he found that his usual tricks merely made the tall brunette flinch and then shrug them back off, was to involve his older brother.
What this imaginary Dean did varied greatly on the situation. Sometimes he would stand by Sam's bed and yell and curse at him, telling him just how much he despised his little brother and how he'd wished Sam was never born. Other times Dean would sit on the floor by Sam's bed, driving jagged pieces of glass into his arms until they were nothing but mutilated slaps of meat, leaving the blonde sitting in a pool of his own blood with a manic cackling coming from him.
Other times Dean would fuck him, straddle his brother's lean hips with his freckle-dusted thighs and grind and twist against him, hips rolling like those of a stripper and hands skilled as those of the most expensive prostitute. His face would remain emotionless, blank and unaffected, as he rode his little brother to completion, wringing orgasm after orgasm out of the tall brunette until he was flaring hot with bright pain, begging the spectral image to stop and leave him be.
Lucifer would always watch from the corner of the room, perched on his chair like usual and occasionally make filthy comments to Sam about this and that. Even if the shock value of dead foetuses and slit wrists had disappeared, he could always make the brunette cry out whenever he let Dean pleasure him. It was the sweetest torture there was and Lucifer enjoyed watching every second of it.