This song is so sad. ;-; This song is actually based off of a Japanese folklore called the crane maiden. The story tells of a crane who was rescued by a man. The crane visits him in human form then they fall in love and get married. One day the man falls ill and the couple didn't have any money to pay for a cure. So the crane weaves clothes from her own feathers to get money and in the end, it seams like they both die. Check out the video: https://www.youtube.com/watch?v=MnKoiKTigoI
All the credit goes to Unichu and this song is by Kagamine Rin and Len.