Seraph of the End: Vampire Reign

A virus killed every adult on Earth and left the surviving children easy harvesting for vampires, who herded the remaining humans into underground cities and bled them in exchange for shelter. Yuichiro Hyakuya escapes the cattle pens after his foster family is butchered for trying to do the same, and resurfaces in the human resistance — an army that has weaponized the same magic the vampires use against itself. He swears a private oath to find every vampire who was there that night, and a second one, much later, when he learns his closest friend made it out too.