Mystara

The Original D&D campaign setting developed Dave Arneson that takes place on the world of Mystara.
The planet has two moons that follow different orbits. Matera which has a monthly lunar cycle and equatorial orbit. Patera which has a 84 hour long lunar cycle and polar orbit. Unlike most worlds, the deities of here are known as Immortals. Many of the older Immortals have forgotten how they reached a divine status, but almost of of the younger immortals had to be sponsored and live as mortal as they attempted divine status. The immortals are divided up between five spheres; Energy, Entropy, Matter, Thought, and Time. Although they may have their own pantheons, Immortals from each of these sphere tend to stay closer together.

The planet has a Hollow World, an interior which is designed to contains the cultures and creatures that have gone extinct. The Hollow World is mainly overseen by the Immortal Ka, who sole function is to make sure things endure. Within the Hollow World, each nation is not aware that there time has passed and strangely enough any warlike behaviors of expansion or war are no longer part of the culture, defense is always in mind. Likewise there are a number of floating islands that orbit around the Hollow Worlds Sun. On these islands, certain culture have been isolated and very rarely venture to the surface below.

The majority of the early campaign tool place in the Kingdom of Blackmoor, and dealt with two major world changing plot threads. The Egg of Coot and The City of the Gods. The Egg of Coot is a feared entity, who is powerful and control a great deal of magic. It is malign in nature and seeks to tale control of the world. The City of the Gods on the other hand, is the remnants of a crashed spaceship of technologically advanced beings from another world. Its engine core is damaged and is unstable. Eventually some adventurer's interfered with the core, which in turn had devastating results on the planet.The Egg of Coot reacting to this danger, transposed much of Blackmoor along with itself to the planet of Oerth.

Centuries later, the people of Mystara survived and flourished. A number of great cultures and civilizations developed on the three continents, and numerous islands found on the world. Mystara was set up to be the original world of the D&D Universe.