Tarn is one of many worlds which drift in the Cosmic Sea. It is a flat plane, with seven known continents: Norwyn, Vaeheim, Sumner, Arden, Ysuna, Zai, and Lokara. These land masses are sundered from each other by several lesser seas, but encircling them is the Great Sea, which stretches to the borders of the world, to the Teeth of Ice.
Beneath the surface of the earth is a great cavernous Underworld, and though the light of the sun has never touched these lands, many creatures great and small have made their home here.
Beneath the Underworld, as far below the Overworld as the crown of the Aether is above the dwellings of men, is the Netherworld, which lies at the Roots of the Earth, beneath floor of the Great Sea. It is said that here, the Vorgar Lords are held in eternal judgment, after their defeat in the Battle of Mal'Gazar.