Owari no Seraph

A mysterious virus wipes out most adults, allowing vampires to emerge from hiding and gather surviving children underground as a source of blood. Yuuichirou and Mikaela Hyakuya live with the other children from their orphanage as a family and dream of escaping captivity together.

Their attempt ends in a massacre from which only Yuuichirou seems to escape. On the surface, he finds a human army fighting vampires with cursed weapons and eventually joins a vampire extermination unit driven by revenge. Over time, he learns that the war is not simply divided between good humans and evil monsters. When signs suggest Mikaela may still be alive, Yuuichirou's goal changes from vengeance alone to recovering the family he believed was gone.