Seraph of the End

In 2012, the world allegedly comes to an end at the hands of a "man-made" virus, ravaging the global populace and leaving only children under the age of thirteen untouched. At the same time, the vampires emerge from the recesses of the earth, followed by age-old horrors of the dark thought to be only myth. They sweep the earth and claim it in a single violent stroke, subjugating the remnants of humanity and leading them beneath the surface to safety. This "protection" comes at the price of "donating" blood to their captors, not knowing that they are vampires.
At the age of twelve, Yuichiro and his friend and fellow orphan Mikaela plotted to escape along with the children in Hyakuya Orphanage. This plan was brought into action by Mikaela, who had been secretly donating his blood to the Seventh Progenitor, Ferid Bathory, in exchange for a map and a gun. However, this plan resulted in every last person besides Yuichiro dying at the hand of Ferid, and Mikaela sacrificing himself in order for Yuichiro to escape and be saved by members of the Moon Demon Company, an extermination unit of the Japanese Imperial Demon Army. Four years later, Yuichiro dedicates his life to destroy vampires and seek revenge against them for killing his "family." At the same time, it is revealed that Mikaela survived thanks to Third Progenitor and Vampire Queen Krul Tepes, who gave him some of her blood to turn him into a vampire. Mikaela does become a vampire soldier right under Ferid Bathory, and plans to find Yuichiro.