私ハ二人ノオ祝イニ (Watashi wa Futari no Oiwai ni)

Background
To you who remains beatuful in death

After fusing into one being Amaterasu(Miku) and Tsukuyomi (Gumi) become to imcpatable to remain one for much long, Amaterasu wants to remain as one being, while Tsukuyomi wants to become her own person once again. After struggling to become the one in control, they end up killing themselves.

This song is the final part in The Sister's Story