Seraph of the End anime

The story takes place in a post-apocalyptic world. In 2012, a mysterious virus swept across the Earth, killing all humans over the age of 13. Vampires emerged into the chaos: they emerged from the shadows, subjugated the survivors (mostly children), and kept them underground, using them as a source of blood.
The Japanese Demon Imperial Army, an organization dedicated to fighting the vampires, emerges to the surface. Humans have learned to use "cursed weapons": blades, bows, and other weapons imbued with demons that give the fighters their strength.
Gradually, the heroes come face to face with the reality of war: battles are brutal, and the lines between "friend" and "foe" blur. As events unfold, new details are revealed about the nature of the virus, the demons within the weapons, and the complex and ambiguous goals of both sides.