From Manga-Heaven:
Long ago, a devil traversed the world leaving nothing but death in his wake. Eventually the demon was slain by the 10 gods who created the world. As the people began to worship the gods, the gods created 10 swords infused with their power to protect the world from evil and peace dawned upon the world once more...
Mirim is a member of the 10 Pillars Sword Guild and one of its elite earning the title of Crimson and the right to wield one of the swords forged by the Gods. Her companion, Ash Grave, is the devil of legend. Why is a swordswoman with divine power to protect the world paired with the demon that is said to be destroying it? Perhaps there is more to this legend than meets the eye...

Itsuki Minamori has fallen asleep in class and dreams about a girl from his past. He is awakened by Haruna a girl who apparently has a crush on him, but her friend Chihaya Yuuki tells her to stay away from this boy. He explains that he's spent his life being followed by evil ghosts and during the exposition, another one shows up.