Brothers Always
"Swear on it," Regulus said, as seriously as a ten-year-old could.
Sirius sighed deeply, but withdrew his wand nonetheless.
"I swear upon my noble and most ancient blood that I, Sirius Orion Black, will always honour my brotherly bond with Regulus Arcturus Black. So mote it be," Sirius vowed, pointing the tip of his wand at his wrist.
There was a flash of gold, and a brand appeared on Sirius' wrist, small but distinct in its raw, vivid redness. It was a vertical slit on his wrist, the physical marker of a blood oath. With a solemn nod, Sirius handed his wand over to Regulus, who was still too young to have his own, and the younger boy placed it against his own wrist.
"I swear upon my noble and most ancient blood that I, Regulus Arcturus Black, will always honour my brotherly bond with Sirius Orion Black. So mote it be," he repeated, before letting out a gasp of pain as the brand was seared into his skin.
Sirius smiled balefully at his little brother, who wasn't as accustomed to the wrong end of their mother's wand as he was, and took his wand back as Regulus rubbed his wrist in an attempt to soothe the agitated skin.
"Brothers always," Sirius recited.
"Brothers always," Regulus repeated.
-oathbreaker-
Sirius gasped in pain as he stumbled out of the Potters' fireplace, into the warm arms of Euphemia Potter, entirely independent from the after-effects of the Cruciatus Curse his mother had been holding him under.
The brand on his wrist had split open as he stepped into the fireplace at Grimmauld Place, bleeding freely against his cashmere robes. As Mrs. Potter called for Mr. Potter's assistance, he stared forlornly at the bloodstain on the cuffs of his robes, thinking back to the awful events of the past few hours.
Mother had called him into the drawing room, and informed him that he would be taking the Dark Lord's Mark that night. He had refused, and she tortured him. Bellatrix joined in once she heard the screaming, while father stayed outside, watching dispassionately as the rest of the family ate dinner, oblivious to what was going on down the hall.
Later in the evening, however, Regulus had risen from his seat at the table, and mother had called him in, demanding that he participate. He did not object, but his hand was shaking as he raised his wand and, after he cast the first spell, he let out a blood curdling scream, falling to the floor as he clutched his wrist, which was pouring out blood.
Seizing his chance, Sirius had made a dash for the Floo while mother and Bellatrix were momentarily distracted. Father just watched, as though he did not care for either boy's plight, as Sirius escaped through the Floo, while Regulus writhed on the floor.
Both boys would eventually be able to stem the flow of blood, but each was left with a mangled scar on their wrist - the sign of a broken oath.
Brothers always, indeed.