Mundus is the plane or realm of existence that encompasses Nirn, its moons, and attendant Aedric planets. For all intents and purposes Mundus refers to these heavenly bodies alone, as they themselves float in Oblivion. The Mundus is a collaborative construct created by many divine beings. It was first conceptualised by Lorkhan, who then convinced other et'Ada (except the Padomay-aligned et'Ada, i.e., the Daedra) to assist him in its construction. Of these et'Ada, Magnus became the Architect, who drew up the plans for Mundus.
Some of the et'Ada gave parts of themselves entirely to the creation of Mundus. These became the eight planets of Mundus (not including Nirn); they are also identified as the Eight Divines, or Aedra. Some Aedra gave themselves completely to their creation, and became the Earthbones that form the substance and life of the final planet: Nirn (a.k.a. The Arena). Soon after the planets had formed, Magnus became disgusted at what Mundus was, and so left for Aetherius, his passage ripping a hole in Oblivion (the realm of Daedra that surrounds Mundus and covers it from Aetherius). This rip is viewed as the sun, and is responsible for much of Mundus' magical energy. Others soon followed after Magnus; these et'Ada left smaller holes, which became the stars. Some et'Ada did not manage to escape or chose to stay on Nirn as spirits. These et'Ada changed into the Earthbones (Ehlnofey) or simply wasted away into nothingness.
Because of Lorkhan's trickery of the other et'Ada, the Aedra met at Direnni Tower (a.k.a. the Adamantine Tower or the Ur-Tower) to discuss the punishment of Lorkhan. Trinimac then ripped the Trickster's heart from his chest and fired it into the sea with his bow. Its impact rent the earth and created a mighty volcano, now known as Red Mountain. The creation of the landmass that is modern Vvardenfell can be traced to this event. Because Lorkhan's Heart remained on Tamriel, it impregnated Nirn with his spirit (according to The Lunar Lorkhan : "a reasonable amount of his selfishness"), while his rotting corpse remained floating above the new planet as the two moons.