It sounds like you're referring to the kingdom of Aimia.

Aimia was one of the Silver Kingdoms during the Heraldic Epochs. It contained an Oathgate connecting it to Urithiru. As one of the ten kingdoms and a holder of an Oathgate, it likely held one of the ten thrones in Urithiru.

Mentioned:

1. "The Way of Kings" (Book 1):

- In the interlude chapters, particularly Interlude I-7, there is a discussion between a group of scholars about Aimia and its history of isolation.

2. "Words of Radiance" (Book 2):

- In Interlude I-5, a character named Rysn travels to Aimia and encounters the Sleepless for the first time. This section provides some insight into the mysterious nature of the kingdom and its inhabitants.

3. "Edgedancer" (Novella):

- This novella, which is part of the "Stormlight Archive" series, follows the character Lift. In it, Lift encounters a group of creatures known as Dysian Aimians, who are related to the Sleepless and originate from Aimia. This novella provides more information about their abilities and motivations.

4. "Oathbringer" (Book 3):

- Aimia and the Sleepless are not as prominently featured in "Oathbringer," but references to them can be found in various interludes and conversations that touch on the world's lore and history.

Thank you very much for this quick and complete answer. I am impressed by your knowledge on the Cosmere.

You're welcome, and thank you!