Black family backstory. Gen. "The blood of your ancestors brings your honour," Regulus heard through the blood rushing in his ears. "Now you must bring honour to the blood of your ancestors."
"Come in, boy."
Regulus lowered his head as he entered his father's study, glancing back with wide eyes and trepidation to see his most recent nanny briskly departing. He didn't know what to make of this. In all six (and one half, he'd tell most if they asked) years of his life, he had rarely seen his father. Orion Black was, in a word, imposing; he was especially so to a child like Regulus, who wanted no more than to return to his room and play with his brother Sirius, older by only one year.
But, no, today Father had called him. Not Dad, Da, Daddy—Orion would have none of that. Even his mother smiled when the boys attempted to wrap their young mouths around the title, as it inevitably came out adorably mangled.
"Regulus. Look at me." Regulus raised ingenuous eyes to his father's face. "It is high time that you learn your station in the world. You are nobility; your birth deems it." Orion inclined his head slightly. "You are a member of the most powerful and pure family in wizarding history. We did not name you in vain, Regulus; you will be a prince among men."
And despite his general puzzlement, the boy's natural pride realized that this was a great compliment. Princes were those fine men who saved fair women from the forces of evil in the stories the hordes of nannies he and Sirius had acquired had told them before they went to sleep. He certainly wouldn't mind becoming one of them.
"Come closer, my son." At the term of endearment and his pride bolstered, Regulus had no fear of moving around his father's desk. He raised his head proudly, meeting his father's eyes with little fear.
"Good," Orion said, a smile briefly lighting on his face as he watched the flames of confidence stoked in his son's eyes. "Hold out your hand." Regulus held his palm outstretched, and his father drew an ornamental knife from a drawer in his desk. He ran the edge of the blade along his son's palm; the boy flinched but didn't move, his eyes instinctively closing hard to ignore the pain. "The blood of your ancestors brings your honour," Regulus heard through the blood rushing in his ears. "Now you must bring honour to the blood of your ancestors."
Regulus opened his eyes a moment later to see blood glinting along the edges of the wound, and registered dull pain when he tried to close his hand. He could feel his heart beating as though its tremors filled the air around him.
"Toujours pur," Orion Black said. "You are dismissed, Regulus."
Regulus stood there a moment longer, resisting the urge to surrender to the pain and sacrifice tears to the blood, and then he understood. This was the blood he had to honour. This blood was not a sacrifice to honour something noble; this blood was something noble for which he had to sacrifice.
He left, and he didn't let the nanny heal him until he grew weak with the loss of blood. There still remained a scar. He kept it until the day he died.
His hand was never quite the same—Father had had the good grace to not slash open the palm of the hand he favored, ergo the damage was generally unimportant. Sirius, it seemed, hadn't been so lucky. He struggled with the quill as he tried to relearn how to write with a crippled hand, as their father looked on with a mix of stern amusement and pride.
And—of course—despite that he was better off, the nature of sibling rivalry forced Regulus into jealousy. Of course Sirius would have the chance to prove himself; he was the heir, after all.
Sirius was about eight inches through a sheet of parchment now covered with jagged writing: Sirius Black, Sirius (and it was so painfully long) Castor Black, Sirius C. Black, S.C. Black. His handwriting was irreparably changed, Regulus knew. Sirius would never forget the lesson behind that scar.
Regulus watched him for a few minutes, then sat beside him at the desk, his feet dangling from the chair. "What did he mean?" he said.
Sirius gritted his teeth, signing the last of the signatures in large letters harshly across the page. He ripped off the parchment and tossed it aside. "The Mudbloods," he said. "He meant the Mudbloods."
"...Oh." Regulus had heard the term tossed around in idle conversation, but now it seemed more important, now that it was relevant to him. "What about them?"
"They're poisoning our l—past," Sirius said matter-of-factly. "We'll all die off because the Mudbloods are covering the world in dirt."
A strange image came into Regulus's head, of cackling hags with shovels of dirt—disturbing, so he didn't pursue it further. "How do you know?" he said.
"Talked to Andromeda," Sirius said. "She told me loads."
"Their blood is brown, that's why they're called Mudbloods. And they're spies for the Muggles. And they steal our magic."
Regulus had only seen Muggles at a distance, but that was inevitable, they did live in London. He'd never seen a Mudblood, at least knowingly… "Are they Muggles?"
Sirius shook his head. "They're freaks."
Regulus stared soberly at the desk. "But we're normal," he said, uncertain.
"'Course we are, Reggie." Sirius grinned. "Of course we are. We're pure."