Note: This song is known as Requiem of the Endless World in western fandom, but Requiem of the Self-repeating World is more suitable. This tradition comes from Motokokusanagi's mistranslated title.
"Meguru Sekai no Requiem" was first uploaded on December 19, 2012 as the third and final song of the Synchronicity Series. At the same day, the album "EndlessroLL" was also released, with Meguru Sekai no Requiem as one of the tracks in Yamashizuku's first major debut album. On December 29, 2012, the song reached Nico Hall of Fame (exceeding 100,000 views).
Suzunosuke have released several works in relation to Synchronicity. Several of which are Colorful 2 (with one of it's story uploaded to Suzunosuke's Pixiv account one day after the song Meguru Sekai no Requiem was uploaded to Nico Nico Douga; also the full color book "Synchronicity ~Kioku no naka no komori uta~" to be released on December 31, 2012, Comiket 83.
The song ends the three years cliff hanger of the series, with Rin and Len finally reunited. Their wishes completely opposite to each other, they faced their fate, singing to the dragon so it could "sleep for an eternity" even if it mean giving their lives away. However, instead of both of them dying, they live, successful in sealing the dragon away forever. Whatever happens to them next is up to the interpreter.