私ノ七ツノオ祝イニ (Watashi no Nanatsu no Oiwai ni)

Background
Watashi no Nanatsu no Oiwai ni is the first song in The Sisters' Story, a series by masa.

The story tells of the twin sisters Tsukuyomi (Gumi) and Amaterasu (Miku) who used to be one entity but got split into two. Amaterasu ends up stabbing herself and Tsukuyomi to death, believing it will make them whole again.