Otherworld is a demesne formed by Merlyn and the Vishanti that exists coterminately with the home of the Fae Realms. Merlin created a landmass in Otherworld called Avalon, and imbued this dimension with a harmony that bridged the great distance between mortal realms and Tir na Nog. The two landmasses closely resemble the geographic relationship between England and Ireland. Otherworld is closer to the roots of Yggdrasil than any other demesne and contains many hidden paths to and from the Astral Plane to Earth.
Otherworld is ruled by Morgan le Fay. The inhabitants of Otherworld are primarily warriors from Tir na Nog's Courts of Faerie, specifically the vast armies of the Unseelie Court. The armies of Queen Mab stand guard at the Outer Gates, protecting reality from the monsters of Nullspace. The smaller garrison of Seelie Court warriors in Otherworld are there to protect the rest of reality from Mab.
Gemworld is adjacent to this space and the harmony of that realm causes it to bleed into Otherworld periodically. Strict treatises between the occupants of Gemworld and the Vishanti allow this permeable border with minimal impact to either realm.