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 Sister's Story, a series by masa.

The song introduces the two main characters of the series, the slightly-older Tsukuyomi and the slightly-younger Amaterasu during their shared seventh birthday, establishing their familial bond of being fraternal twins. While the two are very close, a petty dispute and accumulated hatred causes Amaterasu to kill Tsukuyomi by a blow through the abdomen with a staff.undefined

While the illustration of the two sisters being impaled on the same staff may be misleading it can be inferred that it serves as metaphor for the consistent theme of the two "being one" as well as their eventual shared pain and confusion.