The Church worships the Guardians of the Light. The main foe to the Church is the mythical Novaki, demons which are currently banished to the Plane beyond the Void, but whose influence still lives on in this world. The Church, in areas where it is powerful, deems users of arcane magic to be witches and causes them to be hunted and executed or imprisoned. The Church has the greatest foothold in Celdun, where it is essentially a state religion, but has spread through most of the human populations of Aithar.
The Followers worship the Novaki, claim that they are the Creators of Aithar and are imprisoned by the Guardians of the Void. The Guardians were creatures of the Void between the Worlds who haunt that inky darkness and feed upon the energy of living beings. Starved, they wait there.
In this mythos, the Novaki carefully construct barriers around the worlds they create to prevent the Guardians from entering any worlds that they create, but in doing so, they also must seal those worlds off from themselves and other extraplanar entities as well. Now, the Novaki have been driven back to their home plane by the Guardians, and the Guardians have begun leeching vulnerable worlds.
Aithar is one of those worlds. Many centuries ago, a great war that began, and the war encapsulated the known world. In one of the greatest battles, the Battle of Mount Galindre, two nations and their largest armies, including entire battalions of mages, fought over a region in the pacifist land of Galduun, where lay hidden one of the world's Lodestones, or sources of magical power. In the fight, opposing mages attempted to tap the Lodestone with Legacy Magic, and in their struggle, it is impossible to know what happened – only the results.
In the cataclysmic aftermath, Galduun was leveled, turning it into a poisonous wasteland. A Rift was opened straight into the Void, and for a few brief moments, Aithar was vulnerable. And some of the Guardians stepped through. Now they are trapped here - but have an abundance of life from which to feed, and grow far more powerful than their brethren. Their hope as a species is to unlock the secrets of the shield that the Novaki set in place, and open the world to the rest of the Guardians.