https://myanimelist.net/anime/35994/Satsuriku_no_Tenshi13-year old Rachel awakens to find herself trapped in the basement of an abandoned building. Without any memories, or even a clue as to where she could be, she wanders the building, lost and dizzy. In her search, she comes across a man covered in bandages. He introduces himself as Zack and he wields a grim-reaper like sickle.
A strange bond is struck between them, strengthened by strange, crazy promises…
These two, trapped in this strange building, don't know why fate has placed them there. But they will work together desperately to find a way out…