“Even if you regret what you've done, it's already too late”
"Yuuyami no Ayame Uta" and it's answer song "Guren Hyakkiyako", tell the story of a deadly 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 demon and curses him to die along with her, believing hell is the only place where they can be together. Sometime after, the scorned woman comes to them and kills the girl and later the man to make him hers alone, presumably influenced by the girl's curse.