Bellatrix laughed. "Avada Kedavra!"
There was a flash of green and everything went black.
Red. Purple. White. Yellow. Blue. Curses of all colors spewed forth from the dueling brothers' wands, each more ferocious and deadly than the last. The taller, shaggier brother barked out a quick laugh that was anything but amused. "Mother must be so proud!" He ducked a volley of spears and responded in kind with a barrage of bone-breakers. "Finally got the perfect pureblood prince she wanted, didn't she?" A swipe of his wand sent fiery ropes towards the smaller, silver-masked figure. "Shame it had to be the spare!"
The words had their intended effect, for the younger brother flushed with barely restrained anger. "Bombarda Maximum!"
Dirt erupted as the curse struck near the older's feet, shattering his hastily cast shield and knocking him back. A fast disarming and paralyzing spell finished the duel, leaving the younger brother pointing two wands at the older's crumpled form. The older brother watched with livid but resigned eyes. He knew what was coming but was powerless to stop it. "Tell dear ol' Moldyshorts I'll see him in hell."
The faceless figure watched blood dribble down the older's chin. He raised his wand, his hand quivering. He knew what was expected of him.
A second passed.
A minute.
Finally he lowered his wand. He couldn't do it. Even now, he saw the laughing boy who'd read him stories, who'd taught him to ride a broomstick, who'd gone a whole week speaking only in puns just to annoy Mother. Not the angry man sneering up at him.
"Waiting for Bella to come hold your hand, Regulus? So much for-"
Regulus hit him with a Stunner.
Regulus never wanted it. The war, the deaths. It was all so pointless, so Muggle. Now kin fought against kin, families were torn apart, and he was too weak to slaughter his own brother.
Yes, Regulus made a terrible Death Eater.
"Black Magicae Familia." The whispered words echoed in the heavy silence, broken only by Regulus's heavy breathing and the dripping of blood. Carefully, carefully, he dipped the tip of his wand into the rapidly growing pool of blood, fresh from his own wrist. Then he knelt by the pale form of his older brother, gently turning over his right hand and tracing a symbol on his inside wrist. Algiz, the symbol of eternal protection. "Vitae Praesidium."
The sigil flashed white and sunk into Sirius's skin, leaving only drying blood. Regulus let out a shaky breath. There. It was done. No matter what happened now, he knew Sirius-reckless, foolish Sirius-would be safe. He'd done all he could. Now he could face the music.
Shaking hands made it difficult to fully heal the gashes in his wrist, so he had to settle for binding the wound tightly. He dug an emergency Blood Replenisher from the folds of his cloak, downing the potion in one gulp. Not that it would matter. Soon nothing would matter anymore. Regulus found that thought strangely reassuring.
"Kreacher."
CRACK!
"Master Regulus summoned Kreacher. Does Master need help disposing the filthy blood traitor?"
"No, Kreacher. I need you to erase his memory. Only the last twenty minutes. Please."
"As Master Regulus wishes."
A snap of the house elf's fingers and the encounter is removed from Sirius's mind forever. Another snap and the blood-so much blood-is gone. Kreacher sees the state of his master's wrists, eyes widening. Regulus interrupts him first. "Not now, Kreacher. I need you to apparate us both to Grimmauld and ready one of the owls from the Post. Can you do this for me?"
"Yes Master Regulus."
CRACK!
A few minutes later Sirius Orion Black awoke in an abandoned field with a sore wrist and absolutely no idea how he got there.
Regulus stood over his precious calculations, his meager life's work, debating. Not all prophecies came true, and now that the Dark Lord had been informed it was unlikely the Potter brat would live to adulthood. Unless…
He hated the Potters. Loathed them. Wouldn't blink if they were massacred in front of him. But for Sirius, they were the family he never had. James was the brother Regulus had never been.
Regulus folded the parchments, depositing them in a slim envelope bearing no signature. Their brat would have a chance, abet a small one, if the necessary sacrifices were made.
"Deliver this to Potter Manor. Take no return messages."
The Post owl gave him an accusing glare and flew off into the night.
This was it.
This was the end.
Regulus looked down at the sickly green poison and smiled.
Far away in a different time and place, Sirius awoke in the Hogwarts Hospital Wing, a scar carved into his wrist.
So, the general idea of this is that Regulus discovers a way to block the Killing Curse with sacrificial magic. He performs a ritual that will allow his death to protect Sirius from the Killing Curse, just as Lily's death protected Harry. If anyone doesn't think this idea is an utter heap of crap, I might turn this into a twoshot showing what happens after Sirius survives.