The world consists of nine major realms. Each plane is a part of Yggdrasil, the World Tree, growing like apples from its branches. The planes are connected by the Bifrost. Some of these planes are almost endless, others small enough to traverse on foot, yet others are not easily measured by distance. There are also several other, smaller realms, and a myriad of demiplanes created through magical means.
Midgard, the Material Plane
Midgard, or the Prime Material, is the largest plane. It lies at the center of the world. Despite being the most well-explored plane, it still holds many secrets.
The Underdark
Beneath the Material Plane, a vast system of labyrinthine caverns, tunnels, and chambers sprawl out. Despite not being a true plane, it is so alien from the surface world that it might as well be. The denizens of this place form a separate ecosystem from the surface-dwellers, and only rarely do these groups interact.
The Inner Planes
The Feywild and the Shadowfell form the Inner Planes. To travel there, it is enough to find a crossing where they intersect. The inner planes are hollow copies of the Material Plane. Features of the world are repeated in all three of the planes, although strange and twisted.
The Feywild
The Feywild, or Alfheim is a plane of eternal twilight which can be described with two words: beautiful and dangerous. Here, time has little meaning and the land exists in a constant balance of crepuscular doldrums. The first inhabitants of the Feywild were great animal spirits that were spawned by the vanir gods.
The Shadowfell
The Shadowfell, or Svartalfheim, is a dark and largely evil plane of undeath and suffering. Those who draw upon its magic leave some of their souls behind as payment. In the far outlands of the Shadowfell, the great wolf Fenrir roams. Odin has foreseen he will come to Midgard and do battle with the gods when Ragnarok comes.
Bifrost, the Astral Sea
The Bifrost bridge, made from rainbows, connects the Inner Planes and the Outer Planes through the aether. It is not a bridge in the traditional sense, but rather a conduit through which powerful magic can transport both creatures and objects. Bifrost is connected to the weave that powers all arcane magic.
The Elemental Planes
In a ring surrounding the inner planes lie the Elemental Planes. They form a circle, with water and fire on opposite ends, and earth and air also opposite each other.
Rarely, the innate magic of the elemental planes lashes out through the Bifrost and forms a portal to the Inner Planes. This can lead to the incursion of elementals. Sometimes these portals can become permanent in locations of elemental power, such as a volcano.
On the Elemental Planes, distance and direction does not have the same meaning as it does in the Material Plane. All these planes have a defined center, and the further you go from there, the wilder the elemental forces becomes, until you reach a different elemental plane. These areas are strange and dangerous, but contain powerful magic. For example, at the border between the Plane of Fire and the Plane of Air are the Howling Dunes, a land of scorching hot desert winds.
The Plane of Fire
The Elemental Plane of Fire, or Muspelheim, is a hot desert of black sand dunes, lava rivers, and volcanoes. The warmongering leaders of the fire plane raid other planes for slaves and glory.
The Plane of Water
The Elemental Plane of Water, or Vatnaheim, is an endless ocean. There are settlements here which lie on the shallower parts of the seabed. Coral reefs, kelp forests, and deep ravines hold many dangers. Chief among them is Jormungandr, the World Serpent, who will come to Midgard during Ragnarok and devour all life in the oceans.
The Plane of Air
The center of the Plane of Air, or Skyfarheim, is a collection of islands, floating on clouds in an endless sky. The largest island, Aetheria, lies at the center of the plane, and is the seat of Emperor Yan-C-Bin, who rules over a myriad of princes and nobles, all vying for power in an intricate game of favors and counter-favors. All beings who make this plane their home must know their place in society, or risk offending those with power.
The Plane of Earth
The Elemental Plane of Earth, or Myrkheim, consists of a great cavern system, sprinkled with swamps, bogs, and small cities. It is the least civilized of the elemental planes. Further from the central chambers lies unexplored tunnels, filled with gems and riches but also dangers.
The Outer Planes
Vanaheim
Above the inner planes lies Vanaheim, where the gods live. While they spend most of their time here, they have the power to appear on other planes as well, either in person or through intermediaries.
Helheim
Far below the Material Plane lies Helheim, a land of ice and darkness, and the realm of Hel. It is where souls go when they die, if no other afterlife claims them. It is a frigid expanse of glaciers and stinging winds, where devils and demons run rampant over the souls who are lost in the darkness.