Thirteen is too young to gain eight years of memories that might come to pass--especially when you don't even know what to do with yourself in the present. But when the future is that bloody and dark, do you have any choice but to try to change it? He remembers the absolute agony of the doctors trying to save him, which finally sent him into blissful unconsciousness— —except he doesn’t remember any of that. Tsuna’s last memory is of putting his head down on his desk and falling asleep to the sound of the teacher’s droning, thirteen years old and tired of not understanding a word, not of being twenty-two and beloved and dying.

Tsuna is not quite human, neither are his guardians. Magic and flames can have some odd interactions.

Tsuna and his guardians died in a blaze of glory. Then they woke up, slowly, one at a time, in a different world that was the same in all the ways that really mattered. Or the one where Tsuna and his guardians are reincarnated into the class below the Konoha rookie nine.