どこかで聞いた唄 (Dokoka de Kiita Uta)

Background
As a result of the Leviantan Fire Disaster, Irina is is left dismemberd and is on the verge of death. However Seth Twiright, her creator, comes before her and transfers her soul into a stuffed cat, saving her life. Before leaving, he tells he he never will come back so she needs to "keep expermenting". Irina vows to take out her vengance on Elluka, it having now become her only reason to live.

This song was released on Original Sin Story -Complete Edition-.