Hogwarts: Regulus Only Wants to Study

After transmigrating to 20th-century Britain, he becomes Regulus, the young master of the Black family in the world of Harry Potter.

The bad news is that he knows very little about the original story. He only knows Harry, Hermione, Dumbledore, Voldemort, and Snape, plus a few terms like Hogwarts, wizards, wands, and broomsticks.

The good news is that this is a timeline where Voldemort is still an unknown nobody, and The Chosen One has yet to be born.

Regulus had planned to leave Britain and stay out of the wizarding war, but a prophecy leads him to the halls of Hogwarts after all:

The secrets of the four Founders, Grindelwald's departure from Nurmengard, the revival of the Triwizard Tournament, Voldemort's rapid rise, the Order of the Phoenix's covert preparations...

But at first, all he wanted was to study hard, quietly research magic, and find a way to live a little longer.