A/N Written for "Hogwarts School of Witchcraft Wizardry (Challenges Assignments); the Insane House Challenge (672. Dialogue "No, you promised you wouldn't leave me."); the 365 Prompts challenge (243. Relationship-Siblings) and the Daily Prompt (Hurt/Comfort).
For the first time in centuries, Grimmauld place was completely silent. Usually the house was always creaking, as old houses do, and sounds of Kreacher cleaning could always be heard even when everyone was asleep. It was if the house had decided that after Walburga Black screeching at the top of her lungs for the past three hours, no more noise was needed. The eerie silence unnerved 15 year old Sirius Black out more than anything else could. In order not to draw any more attention to himself, he very quietly made his way into his room and looked around at the various Gryffindor and muggle paraphernalia he had begun sticking to the walls since his first summer after beginning Hogwarts. Thanking Merlin that the Trace was obsolete when he was inside wards that enveloped the ancestral Black family home, he pulled out his wand and all of his clothes, quidditch equipment and various other important belongings swarmed into his school trunk. After casting a quick look around to make sure that nothing he cared about was in any cupboards or drawers, he magically lightened and then shrunk his trunk before walking out of his room without a backward glance.
Silently creeping past the study in which his father would be sitting, unaware and uncaring of the argument his darling wife and heir had gotten into, Sirius wondered how long it would take his father to even notice he was gone. When he caught sight of the front door he quickened his steps, desperate to get out of this house and away from his dark magic loving, Voldemort obsessed parents. The old cow would have another thing coming if she thought he'd ever go anywhere near that maniac for any reason other than to attempt to put him in the ground. Two steps away from the door, Sirius felt somebody step out from the shadows behind him and he whipped around, wand ready to defend himself from whatever hex his mother would throw at him this time. Instead he faltered when he saw his younger brother standing before him, wide eyed and scared. Sirius took a step towards Regulus to explain, but was taken aback when his baby brother stepped away from him and a look of anger crossed Regulus's face.
"No, you promised you wouldn't leave me," he whispered angrily, not wanting their mother to hear them any more than Sirius did, "you swore you wouldn't leave me alone with them."
"Reg you know why I can't stay," Sirius implored his brother to understand, "I've put up with a lot of things in this house, this family, but I won't serve that madman." Sirius took a deep breath before asking a question he already knew the answer to, "Come with me Reg, please."
"I can't"
"No," Sirius whisper shouted, "you won't." At the look of regret and pain he could see on his brother's face, which was most likely mirrored on his own, he calmed himself and whispered "you don't have to join him Reg, no matter what they say. Come find me when you're ready to fight back."
Sirius turned away from his brother, unable to look at his brother in fear of breaking down and reached for the door handle before saying "you'll always be my brother Reg, no matter what that cow does to the tapestry." Sirius quickly opened the door and slipped out before closing it as quietly as he could, leaving his life as a Black behind him.