Nare no Hate no Bokura

Nezu was looking forward to his elementary school reunion—after all, his class was united by unbreakable bonds. But when his former classmate, Mikio, reveals that this reunion is a very real matter of life and death...suddenly Nezu finds those bonds put to the test.
(Source: MyAnimeList)
~~ Adapted from the manga series "Nare no Hate no Bokura" (なれの果ての僕ら) by Utsumi Yae (内海八重).