秘蜜〜黒の誓い〜 (Himitsu ~Kuro no Chikai~)

Background
The song was uploaded to Nico Nico Douga on April 5th, 2010 and has gained over 800,000 veiws. Its YouTube reprint, which has surpassed the original, has gained over 900,000 views.

The song tells of a heartbroken angel (Rin) that stumbles upon a girl (Miku) who the angel instantly becomes in love with. But, love between an angel and a human is forbidden yet the angel yearns to find a way for them to be together. The angel then cuts off her wings, and disappears. Afterwards the girl (Miku) meets a mysterious boy (Len) who was the angel from the beginning who is now human, and falls in love with him at first sight. Although she is to be wed to another man and she then left with him. After, he proposed to her. But, another angel who had loved Rin, filled with envy and anger shot the girl and killed her. The now wingless angel sacrifices herself to bring the girl back to life, leaving one feather.