Seraph of the End: Guren Ichinose: Catastrophe at Sixteen (manga) 4

Desperate to save his retainers and friends, Guren makes the ultimate sacrifice and gives up his humanity to accept the demon in his cursed gear, thereby transforming himself into a demon as well. His newfound demonic powers help him protect the people important to him, but the victory is short-lived. Hours later, the Imperial Demons' presumed heir, Kureto Hiragi, takes all of Guren's family and their followers hostage and presents Guren with an impossible choice: track down and kill his childhood love Mahiru Hiragi within a month, or watch as the Hiragis execute everyone in his clan--starting with his father.

Left with no more to go on than cryptic messages about a secret research project that could destroy the world and whispers of an underground city of vampires, Guren must confront the most consequential choice of his life. And all the while, the clock counting down to catastrophe is ticking...