I, A Grim Reaper, Battle The Heavens!

In the summer of 1978, Tohru Honda, an eleven-year-old boy, is enjoying a peaceful afternoon in Japan. As he lays on his bed, he notices two mysterious figures with dark mist and blue mist surrounding them. Curious, Tohru decides to investigate and finds himself transported to feudal Japan, inhabiting a different body but retaining his appearance. He discovers he is now Ankoku Tenshi, a member of the royal house Kageuta.

Tenshi learns that this world is a part of the underworld, where reapers live. They collect souls to become more powerful, but so do the angels.

A war what has been going on for the dawn of time, how will Tenshi raise while trying to keep the peace of the three realms.