Tenjou Yuuya has been a bullied boy since the past.

He lives in the home of his beloved grandfather while he goes to school. As usual, he receives a harsh bullying, and he takes an extended absence from school to have some time to heal his wounds.

While on this long time of absence, he takes the opportunity to clean his grandfather's house and goes to a room he never went before, where his grandfather keep many different objects that he gathered from his travels around the world. While he was arranging the objects, he found a door not on a wall among the objects.

Out of curiosity he opens this door, what he found on the other side was...

As the Soviet Union marches on Berlin in May of 1945, a secretive group of sorcerers and madmen gathers to perform a terrible ritual. They care nothing for the fate of the Reich. To them, the countless lives lost in the battle are the perfect sacrifice to fuel their spell. After the war, they vanish and are not heard from again, except in rumor. One day, it is whispered, they will return. And on that day, the world will end. Fear the demons in human form. Fear the Longinus Dreizehn Orden.

Flash forward to present-day Japan. Fujii Ren is released from the hospital after two months spent recovering from wounds suffered in a vicious brawl with his best friend Shiro. As Christmas approaches, he tries to rebuild his life, clinging to what brilliance remains. But even that is soon threatened. For the knights of the LDO have returned, and brought the apocalypse to his doorstep. Forced to wield the same unholy power as his foes, will Ren be able to protect what is dear to him from the inhuman might of the Longinus Dreizehn Orden?

From MangaHelpers:

Unknown entities known as "Witches" roam the cities. They exist solely to destroy humans and turn them into one of the Witches. There is a sector in the government known as the Suppression Corps that specializes in dealing with these Witches, who are completely immune to conventional weaponry. Hansel Takamine is the leader of this group, and has an extreme hatred towards the Witches. With his sword imbued with the powers to eradicate Witches, he fights to avenge his sister who was murdered by these monsters. [DrCoke]