The Guardian first appeared in Ultima 7 (The Black Gate), when he tried to use an organization that he convinced some Britannians to found, called The Fellowship, to create a gate for him to enter. The Avatar discovered the plot and stopped him. The former leader of the fellowship, Batlin, fled to Serpent Isle, however the Avatar found and stopped him at the cost of his friend, Sir Dupre's life.
The Avatar was then banished to the world of Pagan by the Guardian, who had conquered it centuries ago. The Avatar eventually defeated the "Titans", who the Guardian had tricked the inhabitants into creating, but ultimately ended up destroying most of their world. The Avatar became the Titan of Ether so that he could return home.
In the final game, the Guardian has established himself in Britannia, and the avatar must cleanse the shrines of Virtue and remove the Guardian's influence. The Avatar eventually defeated the Guardian with a contained spell of Armageddon, which led to his own ascension and the Guardian's defeat.