When a person dies with intense regrets or obsessions, that person may simply refuse to die. It's not a conscious choice, it's a literal refusal by the body and mind to accept death. Once this happens, the person is reanimated as a Shikabane, a living corpse. Now fueled solely by the obsessions that prevented them from accepting death, Shikabane live solely to carry out their obsessions. Shikabane place no value on human life, and their obsessions or regrets invariably are twisted and warped into a justification for killing anyone who crosses their path.
However, there are Shikabane that are created intentionally by the living. They are known as Shikabane Hime. Shikabane Hime are able to retain their sanity and humanity, and are tasked with hunting down and killing rogue Shikabane. Once a Shikabane Hime kills 108 Shikabane, she will be allowed access to Heaven.
This anime follows the exploits of Makina Hoshimura, a Shikabane Hime. Turned into a Shikabane Hime after being brutally murdered by a group of Shikabane, her only goal now is revenge.