秘蜜〜黒の誓い〜 (Himitsu ~Kuro no Chikai~)

Background
Also known as Secret~Black Vow~ or Alluring Secret~Black Vow~, this song was uploaded to Nico Nico Douga on April 5th, 2010 and has gained over 800,000 views; it has also gained attention in the western fandom. It is featured in the EXIT TUNES compilation album "Vocalonexus" and is featured in Hitoshizuku-P's mini album Alluring Secret~Colorless Vow~; with an answer song explaining the girl's point of view, and both are featured in Hitoshizuku-P's album; STORIA. It is also featured in Yamashizuku's debut album EndlessroLL.

The song tells of an angel (Rin), that stumbles upon a girl (Miku), who the angel instantly falls in love with. But, love between an angel and a human is forbidden, and the angel yearns to find a way for them to be together. She reveals herself to be an angel to the girl and dissapears. Afterwards the girl meets a mysterious boy (Len; who is actually Rin. She traded one of her wings to become a human/boy in order for the two to be together), and falls in love. Although she is to be wed to another man, she left with the boy. But, another angel (Kaito) who was in love with the angel, shot the girl and killed her. The angel gave up her last wing to save the girl, sacrificing herself and leaving a single feather.

A novelized version was published on September 24th with a slightly different plot and more characters. A book signing was also available with Hitoshizuku-P and Suzunosuke attending on September 29th.

Novel
A novelized version of the song was released on September 24th, 2012. A book trailer can be seen here.