and I still really love the songs and the stories, especially Alluring Secret ~Black Vow~ (hahaha there's a PewdieCry version)
here's my own interpretation.
it's about an angel who falls in love with a human girl, but the human girl doesn't love the angel, maybe because the girl is already engaged to another guy. as love between angels and humans is forbidden anyway, the angel decides to make a deal with the devil for the girl to love him, so its wings are cut off and it becomes a human boy. the deal is broken if the love between the girl and the angel is broken. on the girl's wedding day, she meets the angel-as-a-human and falls in love with him, as promised by the devil, and leaves her fiance. so the girl and angel-as-human live happily ever after. the end.
BUT WAIT! another angel who was a friend of the angel-as-human believed that the girl had tricked the angel-as-human to sin, so it kills the girl. in this way, the deal with the devil is broken. the angel-as-human sees the dead girl, and believes that "all its acts of treachery should be paid by its death", so it exchanges lives with the girl and disappears, leaving one black feather behind. the girl lives and realises that he was the angel all along. she is a "sinful black-stained bride" (she might have been under the influence of the deal between the angel and the devil and might not have actually loved the angel, I'm not sure and I don't really understand White Vow). it ends with "when the fruit of sin falls into decay, they can meet again", which probably means when they stop loving each other, they might see each other again, but not until then.
now watch it and make your own interpretation :P
lynn