cypsiman2 | Where I watch Episode 21 of Simoun

The Emerald Ri Maarjon is Time Travel. Dominura and Limone wound up in the past, sometime after the civilization that originally created the Simoun had destroyed itself through unceasing war, and they reintroduce the knowledge of how to use the Simoun once more, after Limone convinces Dominura that without the Simoun they never would have met. Also, Aeru's grandfather was the Regina of Choir Dextra, and her music box plays the song of 'Another World', which Dominura sang after traveling into the past. The High Priestess and Onashia want Neviril and Aeru to perform the Emerald Ri Maarjon to find the way to the new world. Neviril begins to hope that Amuria might be alive in another time, which is plainly upsetting for Aeru, and Yun gets a bit of psychoanalysis by Onashia. And elsewhere, the war continues to get worse for the Simulacrum Theocracy, as the Arcus Niger has been taken down, and a squadron of five Ancient Simoun begin their attack.

Or, long story short, CLOSED TIMELINE LOOP, AAAAAAHHHHHHH!