Mad Moon

The Mad Moon was a large, cosmic sphere that orbited the World, the prison of the Radiant and Dire forces, until it destroyed itself in a cataclysmic event. Because of its impact, the Mad Moon ties heavily into the mythology and lore of the World.

Creation, Orbit, and Destruction
Once, the ancient intellects of Radiant and Dire warred with each other endlessly. The two forces, complete opposites in every way, could find no compromise between each other. The Primordials grew tired of their strife and punished the two by fusing them into one object and flinging them into space to drift for eternity. Eventually, this sphere was captured in the orbit of a still cooling world, shining over it as the races came into being. Because of its power, the Mad Moon glowed brightly over the World, competing even with the brightness of the sun.

But the forces inside still fought, and the Mad Moon began to show imperfections. Small weaknesses expanded and eventually the sphere broke apart, raining down material onto the World. The remains of the Mad Moon settled as Radiant Ore and Direstone, pure materials of the two intelligences. The World fell into temporary chaos, though some took advantage of the situation. Abzidian, the Demon Smith, used this time to forge Demon Edge from the raw materials that fell.

Post-Destruction
After the Mad Moon was destroyed, the Radiant Ore and Direstone began to emit pure energy that corrupted the races. They became dependent on it and built civilizations around the material, while the rock provided them with energy, Mana, and life. The Radiant Ore gave off the emission of bright color and charm to the area around it, while the Direstone made the earth appear poisonous and decayed. Neither was neutral in its actions. The energies they gave off were incompatible, and interfered with each other. Eventually, the followers of each of the forces became aware of this, and sought to destroy the opposite stone.

Trivia
This may also be a reference to the Legend of Zelda game, Majora's Mask where there is a moon like it.
 * Lamprey, a Valve Developer, compares the radiation emitted by the Radiant Ore and Direstone to the cosmic radiation in H.P. Lovecraft's The Color Out of Space.