Seraph of the end - Takaya Kagami

After a catastrophic epidemic kills every adult on earth, vampires arise from the shadows to enslave the remaining human population. Yuichiro escapes from captivity and joins the Japanese Imperial Demon Army. But before he's allowed to fight vampires, he has to complete his first, terrifying mission - make new friends with his fellow aspiring monster slayers!

Yuichiro and his friends are kept as vampire fodder in a creepy institution masquerading as an orphanage. One day, Yuichiro's dreams of fighting back are realized when the orphans stage a daring escape. But things go horribly wrong, and his dream is kept alive at a cruel price.