私ト一ツノ心臓ヲ (Watashi to Hitotsu no Shinzou o)

Background
The twin sisters Tsukuyomi(Gumi) and Amaterasu(Miku) vanished on their seventh birthday after Amaterasu's murder suicide with her sister. After meeting eachother by a grave with their names on them, Tsukuyomi wishes to be with Amaterasu a separate people while Amaterasu wants to fuse into one being like they used to be, and evetually manages to overpower her sister.

This song is part of The Sister's Story