This is a piece from the original soundtrack of the anime Shinsekai yori
(also known as From the New World or 新世界より).
The arrangement was made by
I've only created the sheet music for it.
You can listen to the song on YouTube or download it for free on Sheethost (PDF, MIDI or MuseScore files).
Kage no Denshouka Dainibu (阴の伝承歌 第二部)