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 may have been seen in the vision that Clark had of a future Riung; the latter mentioned talking from “one of the White Sky Towers of Umichoo”.