An ancient inter-timeline empire that once ruled “the corner of reality” that contains the rigasian timeline.
At some point they caused some kind of catastrophe that erased their original timeline turning it into a hole in reality that's now known as the Endless Tower.
It is mostly mentioned by the anagusi and their companions to be the World-Breaking empire.
It was first seen in the visions that Clark and Gaia had of a future Riung; the latter mentioned talking from “one of the White Sky Towers of Umichoo”.
By the end of Arc 2 the Umichooan Empire intervened in the Rigasia/Kesizma War by deploying three people that stopped the Rigasian invasion of one of the core Kesizman pocket universes.
As Arc 3 begins there have been Umichooan delegations in secret talks with the Rigasian government, and additional ones are deployed to Zhongguo, Mus, and Valtshora; this last one finds itself having to deal with an out-of-control Chronomancer, though.