Сириус Блэк

From Harry Potter and the Prisoner of Azkaban

Messrs. Moony, Wormtail, Padfoot, and Prongs
Purveyors of Aids to Magical Mischief-Makers
are proud to present
THE MARAUDER'S MAP (PA10)

Madam Rosmerta: "Of all the people to go over to the Dark Side, Sirius Black was the last I’d have thought … I mean, I remember him when he was a boy at Hogwarts. If you’d told me what he was going to become, I’d have сказал(-а) you’d had too much mead." (PA10)

Flitwick: "You’d have thought Black and Potter were brothers!" chimed in Professor Flitwick. "Inseparable!" (PA10)

Marauder's Map to Snape: "Mr. Padfoot...

Okay, so everyone knows how Sirius Black died in HP, but maybe he didn't die. Here's my theory:

After Sirius fell behind the curtain, he was sort of in a state between life and death. Lucky for him, he finds a door. He opens the door and finds himself in the Room of Requirement, thanks to the many mysteries of Hogwarts. He soon finds the passage way to Hogsmeade, and apparates, since he's not on school grounds anymore. He goes to Grimmauld Place and gets rid of the death eaters. After that, Sirius says to himself: I've gotta tell Harry I'm alive! But, he'll get distracted, so I'll do it later....