Salutations! Thanks for giving this story a chance. I've no idea how far this will go or if I'll even finish it, but the idea bit in and wouldn't let go. So here's giving it a shot!
A Tale of Two Brothers
There was no feeling in the world quite like killing someone you loved.
The grass was stiff and crunchy beneath his knees; there'd been a drought in the Midwest, an intense heat-spell in mid-May that meteorologists had signed away as Global Warming from the very beginning. He knew better, of course; he'd known all along that this was inevitable, all of it, every stroke of heat, every strata of sunlight. That this had been coming for a very, very long time.
In fact, Lucifer had been banking on that.
The battle hadn't lasted five minutes after his brother had stepped into the cemetery, wearing that meatsuit that Lucifer despised. It had come as shock, granted; for all of his preparations and carefully-laid plans, he hadn't thought it would actually come down to this. And he'd known from the steely resolve in his brother's eyes that trying to wheedle out of the fight would only cause more pain in the very, very near future.
Michael was a loyal son. And if Lucifer turned his back, it would all be over.
So they'd met, a collision of pure raw nova energy in the middle of Stull Cemetery, shaking the Earth, their father's masterful creation, to its very core. Half a world away, tsunamis erupted as tectonic plates shifted, spitting molten matter sky-high. Lucifer's world narrowed in an instant: just to Michael, just to his brother's smug, self-sure face and Lucifer realized just how badly he wanted to beat that calmness out of Michael. To break him of his blind faith.
Or just break him.
And that was how they came to be, Michael on his back, dazed by a rocking blow to the side of his head that had him seeing stars, tripling into solar systems and galaxies. And Lucifer half-straddling him, one knee pressed into the soft dirt, the stiff grass of Stull Cemetery, the other crushing the aortic vein that ran through the torso of Michael's pathetic human sock-puppet.
"It didn't have to be this way," Lucifer panted. He himself bleeding, from a swollen, blotchy crater of melted flesh on his shoulder where Michael's angelic power had sliced through, carving a trench into the vessel. "We could've had anything, Michael. We could have become our own gods."
"I would rather die," Michael grated painfully. "Then surrender one hair on this human head to be your servant. You're nothing to me, Lucifer; just a pathetic, insipid waste of this planet's air."
It chilled something right to Lucifer's core, to hear Michael say that. To feel the disownment, as hard as any blow that they'd dealt in their brief sparring match, and reaching twice as deep. Past the vessel, and into Lucifer's own being.
"Fine." Lucifer cocked back his fist. "If that's how it is."
A tremor, a separate shockwave moved through him as Michael turned his head aside. Not in resistance, but—defeat? Was the great Michael, the sword of heaven, God's own warrior supreme, giving in?
Or—?
The head, slowly, turning back toward him, and the eyes weren't Michael's for the moment, but deep, agonized seas of feeling; so deep Lucifer thought he could possibly drown in them.
"No." Lucifer hissed, but his arm faltered.
"Please." Michael raised a hand, blood-mottled fingers tangling in Lucifer's collar. "Please, I'm begging you."
"Not this time." Lucifer's first blow landed solid, snapping Michael's head aside. Lucifer hauled him up by his throat and punched him again, slamming the vessel's head with a sickening crack against the hard ground. The body beneath Lucifer went limp, that hot pulse under his knee spurting wildly out of control.
No! No, no, no, please—please!
Lucifer heaved back on the deadweight of his brother, picking his head up by fistfuls of his short hair and slamming that fragile human skull into the ground, over and over again until brainmatter and blood sprayed across the grass.
No—no, stop! Please! Please, stop, please, you can't do this!
But Lucifer had no reason, he had no mercy. He butchered the vessel until apical shards of skull peeled in gummy strands away from his hands. Then, and only then, did he finally let go, and sat back in violent anticipation, knees gripping his brother's hips, waiting for the first buck of protest as the battle continued.
But there was none; Michael was still and ghostly beneath him, blood flecking his vessel's chapped lips. So much blood.
The voice in Lucifer's head was quiet, now, but not in submission; there was a soft, keening anguish filling Lucifer from the inside, banking against everything he was, struggling for control even as he wiped the sludgy matter onto his thighs and leaned over his brother's corpse.
"I win," he rested a hand on Michael's unshaven jaw. "So, I win."
A cacophonic, brandishing force wind-whipped against Lucifer's back, rocking him forward. Before he had a chance to recover, an ancient, soft voice, aged with sadness, spoke behind him. "Oh, how you are fallen, bright and morning star."
Lucifer twisted around, knees pressing into the turf, the wretched instinct of an animal cornered flooding acidly in the back of his throat. "Father."
Father wasn't alone. There was a vessel beside him, a stocky man in a dark, pressed suit who stood with his arms crossed, one eyebrow arched. "Love what you've done with the place, darling."
"Enough." Father said, and that one word was enough to have all of their tongues in silence. "The seals were broken before the appointed time, Lucifer. You've made sacrifices of innocent men to fulfill your private vendetta."
"And look what you did to stop it." Lucifer taunted, gesturing to the mangled, malformed head twisted grotesquely away from him. "You did nothing, like always. Ever since man's first sin, you've always been all about the Master Plan. Are we really all that different, you and I? Both powerful spirits striving for a goal. But unlike you, I'm not sedentary. I got what I wanted."
Father's expression never changed. "Step away from your brother."
With belligerence and reluctance, Lucifer rose stiffly to his feet. He stepped away, and as he did there was a nearly-soundless rush of movement as another angel stepped forward to take his place, kneeling beside the broken body.
"Ah. Castiel. So nice of you to join us." Lucifer wreathed the words with as much contempt as he could manage.
Castiel paid him no mind; kneeling beside Michael, resting a hand on the vessel's shoulder. The concern in his eyes overwhelming, a weakness Lucifer had shrugged off a millennia ago.
"He's badly hurt. Father." Castiel fixed a haphazard blue stare on Father.
"It's in your hands now, Castiel." Father encouraged, and with a harsh swallow that bobbed his vessel's Adam's Apple, Castiel closed his eyes and began to work over the destroyed body of Michael's meatsuit.
"So, what now?" Lucifer demanded, stalking toward Father. "You let the battle go on for this long. Why not let me finish it?"
"This was never your war, Lucifer." Father said. "It was a test. For you and for Michael, for the humans, for each and every one of you. And you and Michael failed utterly."
"That's no excuse for the blind eye you turned. Toward us. Toward everything."
It wasn't compassion in Father's eyes so much as a welling sadness for what had been before. "Never forget, I heard your songs sung for the beginning of time, Lucifer. I know every sigh of your tongue, every strum of your spirit." He raised a hand. "Your treachery was never hidden from me, Lucifer."
There was an unspoken promise in the words that made Lucifer feel, suddenly, like a fledgling. "You can't. It goes against everything—"
"Your freedom has come to an end."
"No—no!"
It was the brightest burst, the most powerful force he had ever known, lifting him first, then stripping him away like a cancerous growth. He was screaming, the vessel screaming with him as they were rent apart, soul-from-soul, and he heard Father speaking the Enochian spell that would crack the Cage wide open again.
Twisting and flailing, Lucifer was dropped through the black, howling slit in the ground, swallowed down in a rush of hurricane wind and vanishing into the aftermath of his own traumatic screams.
"That's my cue. My work here is done." The man in the suit ducked his head politely if a bit arrogantly, and vanished.
Father paid him little attention; he was focused on the glistening orb of silver-white light nestled like an infant in the palm of his hand. Warm and throbbing and breathing and hurt, in so many ways, not fully understanding why it too had been blasted from its human vessel just like Lucifer.
Tenderness washed over him. "Oh, my beloved Michael." Father cradled the angel's faint essence in both hands and looked to Castiel. "What will you do now, Castiel? Now that you've proven yourself before Heaven and before me. Will you return to the ranks of the host?"
Castiel looked to him, with so much love and conflict in his eyes that Father wished he could wipe it away. But in months gone by he knew Castiel had changed; war had turned him from a tenuous, sycophantic devotee to a leader. Trial by fire had shaped Father's youngest son into a vessel of raw strength and self-awareness; and while at times that awareness had bordered on blasphemy, in the end Castiel had stood on the front lines for humanity and fought the losing fight against a powerful opponent.
And he would make his own way.
"I'll stay." Castiel said resolutely, his hand not deviating from the vessel's shoulder. "This is my place now."
"I know it is." Father said. "If ever you need me—"
"I suppose I know where to find you now." Castiel said almost bashfully, looking down. So much unhappy feeling inflamed between them—for a Father who had been absent, for a son that didn't know how to trust yet—but it could be mended, all of it, if given the proper time. "You aren't leaving again, are you?"
Father's reply was to move closer, and cup one hand against Castiel's jaw. "It's over, Castiel. You can rest easy. This war is over."
Castiel closed his eyes for a moment and Father felt the first strains of unsettlement, looking toward the milky-white horizon.
I fear another will soon be under way.
In a shiver of motion, he vanished, leaving Castiel kneeling alone in Stull Cemetery; alone aside from the memories of Lucifer's last tortured scream as he fell.
Alone, aside from the Winchester brothers, lying side-by-side on grass still soaked with Dean's blood.