Directly following the season 4 finale.
Dean stares at Lucifer with something that is lost between terror and awe. Sam shakes, quakes, in his arms, fingers curling into the leather of Dean's jacket and face turned away as he refuses to look, refuses to see what he has done. Dean shelters Sam without thought. It isn't something he is conscious of, his body simply shifts to shield Sam from Lucifer's burning gaze, a bone deep awareness that he's already prepared to fight the Devil himself in his brother's defense.
Lucifer doesn't spare them a look at first. He crouches next to the fallen form of Lilith. He strokes her blonde hair back and bends to kiss her forehead. "My daughter," his voice makes Sam shudder and Dean's ears ring, both clenching their jaws against screams that want to break free, "Thank you."
It is only then that he turns his gaze to the Winchesters. He regards them coolly, meeting Dean's eyes and then flickering his eyes across Sam. Dean's hands curl into fists at Sam's back and he shifts Sam further away, which does nothing but coax a deep chuckle from Lucifer's throat.
Dean wasn't sure what to expect of the Devil, but Lucifer is not a red-skinned demon with horns and a tail. His skin is pale, as if it once shown with a light that had been extinguished into darkness, and his eyes might have once gleamed as bright as Castiel's but they are now dark as the night sky, his hair pure black. His smile is cool and almost polite as he steps towards them.
Castiel appears there, Dean's own guardian angel. His wings are in shreds, his skin marred with burns, but he is there and alive when Dean was sure he would never see the angel again. His eyes immediately settle on Lucifer and he doesn't look away.
Lucifer breathes Castiel's true name out in the language of the angels, his voice causing both Dean and Sam to flinch and cry out. Castiel's eyes flicker over to them but he makes no move to aid them, focusing again on Lucifer.
"Brother," he nods.
"My Castiel," Lucifer's voice is mocking as he speaks Castiel's second name, "How long has it been?"
"A very long time."
Castiel remembers, in shocking detail, the feel of the Morning Star's wings against his skin, his mouth against Castiel's, their Grace burning together, growing brighter and brighter until their love blinded them. Angels purpose, above all else, is to love their Father. Lucifer loved Castiel more than God and he Fell. Castiel denied their love and he was glorified.
Lucifer hums at the back of his throat, an oddly human action, and turns his eyes back to the Winchesters. "Sam. Come here."
It's said softly, almost gently, but there's no mistaking the quiet order. Dean sneers and tightens his grip, even as Sam slowly raises his head. Shaking, the younger human pulls away and takes careful, hesitant steps forward. Lucifer meets his eyes and there's recollection there, a part of Sam that recognizes Lucifer and burns in delight at their reunion.
Sam falls to his knees before the Fallen without thought, his head tilted up and his eyes wide with aw. Lucifer cups his face with both hands and bends, presses his lips to Sam's forehead in a manner that mimics his earlier treatment of Lilith.
"I have been waiting so long for you, my son."
"Father," Sam breathes and Castiel looks away. Dean doesn't say a word, his eyes glued to Sam.
Lucifer looks back to Dean, one hand falling back to his side, the other dragging through Sam's thick locks. "Dean, I want to thank you for taking such good care of him. You will be honored above the rest, kept safe. Do not worry."
Castiel steps forward, "He is mine."
For the first time, Lucifer's eyes flash with fire and Castiel flinches away, drops to his knees and then seems horrified by the action. "You are mine and no one else's. Not your Lord's and not this human's. Mine, until Heaven burns to ash and the both of us along with it. Never forget that."
Castiel, for one moment, believes him and he cries out as his Grace is ripped from him. He falls sideways, shaking and suddenly cold as he lies on the ground. Lucifer's smile is sad.
"I know it hurts, beloved," something in Castiel purrs at the endearment, the same one used before the Fall, before his Morning Star was ripped from him. Castiel remembers, remembers that he was the Morning Star's beloved but more than that, he remember s that he is, that he will forever remain, the beloved. "I know it hurts, but it must. It's been so long coming."
Castiel shakes because it has, it has.
"Sammy," Dean has finally found his voice. He's inching towards his brother, eyes still focused on Lucifer, "Sammy, c'mere."
Sam glances over his shoulder and frowns, like he can't quiet remember who Dean is. Then he smiles and crawls back to Dean, who drops to the floor and wraps his arms around him, holding him tight. "Dean."
Lucifer doesn't seem to mind Sam's abandonment. He's standing next to Castiel, pulling the newly Fallen to his feet and stroking his face. His eyes flicker to Sam and then over Dean before returning to Castiel.
"And the world will burn before us."
Sam smiles, leaning his weight even more heavily upon Dean, "Of course, Father."
Dean ducks his head lower and drags his lips against Sam's temple. Castiel shudders.
"Yes," he whispers, as if he can do nothing else.
And their fates are sealed and the world will burn.