The Old Doll in The Attic (Diabolik Lovers)

Edit from the future: tags are gone bc the story is discontinued at this point and awaiting a reboot.

a rumor in the vampire realm as that the sakamakis had a younger sister.
Ayami Sakamaki. But something happened, and she disappeared.

when yui is there as a sacrificial bride, she discovers an attic. the brothers tell her she is not allowed to go up there. Obviously, she disobeys their orders as she heard a soft melodic voice calling her. as she reached upstairs the voice sang to her, catching her in a trance. When the brothers catch her up there, they see Yui stuck in a trance as the mysterious voice sang.