The Oldest Black

Castor Black hated his family- well except his younger brothers, Sirus and Regulus. Castor was born into a pureblood wizarding family who saw muggles as filth and beneath them. He hated it. Castor hated growing up in a pureblood family who didn't like each other. His parents, Orion and Wallburga and even his cousin bellatrix worshiped the darklord like some sort of god, but Castor, Sirus and Regulus didn't. Regulus was thrusted into it after sirus was disowned and erased from the Black family tree.
Castor Finds a job as a defence against dark arts teacher at hogwarts where he meets Harry potter and his friends along with his young cousin, Draco.

p.s Regulus Black is alive in it and Sirus would not die