"Youtei no Mori" is an original song by Yandere-P featuring Kagamine Rin.
The story is based on Sleeping Beauty. A Princess(Rin) is locked away in a castle in a forest, cursed to sleep forever. One day she sense the presense of a young Prince(Len) entering the forest, and believes he will encouter nothing but struggles and abandon her like everyone else. However he eventually makes it up and kisses her awake.
This song is featured in the ULOG’Be compilation album, Howring.