An ancient inter-timeline empire that once ruled “the corner of reality” that contains the rigasian timeline.

It is mostly mentioned by the anagusi and their companions to be the World-Breaking empire:

It was a long time ago, that one of the old civilizations in this corner of the world grew powerful enough to control the very foundation of time and space.
And they built gates to the worlds that could have been; and there they created a great and bountiful empire that they called Umichoo.
But their wisdom was not as great as their power, and they did not think their actions through. Because great power makes your footprints great, and their power was great indeed.
And in time, their reckless actions broke the very world that was the center of their empire.
In its place there's now a crack in the world. A rift made of nothing that goes nowhere, that they call the Endless Tower.

When Clark had a vision of a future Riung, the latter mentioned talking from “one of the White Sky Towers of Umichoo”.