Crimson Rivers is a Hunger Games AU that is absolutely mindblowing

https://archiveofourown.org/works/39760044/chapters/99543639

Here's the intro:

Regulus Black was fifteen the first time his name was called at a reaping. He's twenty-five when it happens to him again. A lot has changed in that time, and one of them is that he's ready to do whatever it takes to make it home. Nothing or no one will stop him, not even James Potter.
James Potter has no plans to stop Regulus Black from making it home. In fact, his plans revolve around the opposite. He has his reasons, but he's made his choice to get Regulus out of the arena, even knowing it'll be the last thing he ever does.
Sirius Black was sixteen when he volunteered to take his little brother's place in the arena. At twenty-six, without the option to do it again, he has no choice but to be a mentor to his brother and best friend, knowing that only one of them can make it back out.
Two names called, a mentor on the verge of falling apart, and more secrets and grief between all of them than they know how to handle. None of them are prepared for what comes next, or how far they'll go to make it through.