Tarn is one of many worlds which drift in the Cosmic Sea. Its surface is a flat plane, called the Overworld, with two large regions, Faeheim and Djinnheim. These land masses are sundered from each other by the Lesser Seas, but encircling them is the Outer Sea, which stretches to the borders of the world to the Teeth of Ice.
Above the earth is the Aether, in which the Sun, Moon and Stars are located. The Firmament stretches like a vast, opaque dome above the Aether, insulating the world from the Cosmic Sea, which is not visible from within Tarn.
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, including the Gnomes and the Deep Elves.
Beneath the Underworld, as far below the Overworld as the Crown of Heaven is above the dwellings of men, is the Netherworld, which lies near the Roots of the Earth, beneath floor of the Outer Sea. It is said that here the Vorgar Lords are held in judgment after their defeat at the Battle of Mal'Gazar.