Yuuyami no Ayame Uta / Guren Hyakkiyako

History
"Yuuyami no Ayame Uta" and it's answer song "Guren Hyakkiyako", tell the story of a love triangle.

A man(KAITO) once had a childhood friend(MEIKO) and promised to marry her. However when they had become adults, the man left the woman for a naive girl(Miku), turning woman angered and heart broken. The naive girl however realizes he's cheated on another woman with her rather than been with her and her alone. She then transforms into a golden demona and curses him to die. Sometime after, the scorned woman comes to them and kills the girl and later the man to make him her alone, presumably influenced by the girl's curse.