Among the numerous illegal activities the Black family were involved in, this particular tradition took place when one came of age, and began the process of becoming an illegal animagus.
Of course, Sirius Black doesn’t wait seventeen years. He befriends a quiet boy whose eyes are too old and have too many secrets, and when these secrets are revealed, it is Sirius who knows what he needs to do. It is Sirius who steals heavy books from the section of the family library cloaked in dark spells. It is Sirius who recruits two other boys into his Black family tradition, only it isn’t just tradition anymore, and this is a different kind of family.
Walburga Black spoke with poison her whole life, and occasionally she killed with it too. A second cousin who put his hands where they were not granted. A house elf who heard secrets not meant for her ears. A healer who failed to save her daughter, barely two hours old and two years before she named her son after the brightest star in the sky. The bite of a black widow is not usually fatal to humans, but Walburga had always been a particularly venomous person, as her husband would often rage at her. She would rage back and remind him what spiders did to their mates once they had no use of them.
Orion Black wore the curved horns of a ram like a crown of armour, but they were useless when protecting his family name, the name he married his own blood for in order to keep pure. He preferred brute force over reason, which was effective when used to inflict fear and pain. Yet fear and pain were weak motivators when it came to blood traitor sons and crazed spouses. Orion was the leader of a flock scattered far across a field, and he did nothing when the wolves came howling.
Ten months after Regulus Black turns seventeen and ten months after the Dark Mark is branded in to his skin, he transforms into a sleek black cat in the middle of Grimmauld Place, surrounded by men in masks. Of course, Sirius hears about it eventually. He jokes how they always fought like cat and dog, but cannot bring himself to laugh. In many cultures, black cats are considered a bad omen, guardians of death. A year after he transforms, Regulus Black is drowned in a cave by an army of the dead, where he guards them forever.