Pagan is another planet, entirely separate from Britannia. It lies far across the Ethereal Void. It is a world which was originally peopled by the Zealans. In time, however, The Guardian found Pagan, and began telling the Pagans about a being he named "The Destroyer" which was coming, and for which they needed to prepare. What he didn't tell them was that he himself was The Destroyer.
Some Zealans began following The Guardian, and began calling themselves Pagans instead. Then The Guardian brought the four Titans to Pagan. The Titans convinced the Pagans that they needed to eliminate the Zealans. Following a great war which destroyed much of the world, the Pagans won.
The Guardian then left Pagan. The Titans convinced the survivors of the war that it was they who had defended them from The Destroyer, and demanded worship. The Pagans agreed, but convinced the Titans to stay in four sacred areas, one dedicated to each Titan.
Eventually, each Titan agreed to share some of its magical powers with certain select Pagans, upon which four of the five schools of magic on Pagan were founded.