Aon is the name of the fantasy universe created by Joe Dever, in which the popular fantasy gamebooks series Lone Wolf is set. The universe contains magic which can be used for good or evil, and has gods representing these: for good, Kai, God of the Sun and Ishir, Goddess of the Moon; for evil, Naar, God of Darkness. It is a universe similar to our own, but there are also known Planes of Existence existing outside of it.
Aon contains many worlds, and all but one are ruled by good or by evil; Magnamund is the balance-point, the last world on which good fights evil, and is the world featured in most books. Outside of the universe are the four planes of Earth, Air, Fire and Water, and also the Daziarn where God-like creatures reside. The Daziarn is surrounded by the Plane of Darkness and the Plane of Light.
The Legend of Lone Wolf series of novels by John Grant describe the universe in more detail; Hunting Wolf in particular deals with visiting worlds other than Magnamund in the battle between the good wizard Banedon and the evil wizard Vonotar. The character Alyss from the Legend series regularly visits the Planes. The World of Lone Wolf (Grey Star the Wizard) series of gamebooks written by Ian Page are set both on Magnamund and the mysterious Daziarn plane.
For all eternity, an unceasing, timeless, formless war had waged between the Lords of Good and Evil. This war could never be won, as there was nothing to conquer and no standard by which victory could ever be measured. After countless years, Goddess Ishir, most prominent of the Lords of Good, secured a truce with Naar, the King of the Darkness. Ishir and Naar created a vessel to represent their truce, into which they infused their powers of life and death, creation and destruction. This vessel was Aon, the 'Great Balance'. To the surprise of the gods, from this vessel grew the physical universe with its myriad worlds and life forms.
The Lords of Good and Evil sought to control Aon. With their conflicting desires and a tangible battleground, their war began again in earnest, but with a clear purpose - the domination of Aon. Whichever side conquered the physical universe would become the victor, and the other side would be banished to the void. One by one the worlds of Aon were claimed by Good and Evil until only one free world remained - the world of Magnamund.