Guardian

The Guardian is the main adversary of The Avatar during the Age of Armageddon. From where he comes is not known (thanks to Ultima IX). The Xorinite Wisp describes the Guardian as a being from another dimension, and calls things like egotistic, megalomanic and cruel his character traits. Added to this is the fact, that the Guardian has very powerful magic, an almost limitless amount of Mana, psychic powers and a very high intellect, with which he creates devious plans.

The Guardian conquers worlds, either though lackeys on that world, like on Tarna, or, with brute force, like he did with Pagan. If a world resists him far too much, he destroys them, as he had done with Anodunos. If the Guardian enters a world outside of his dimension, he becomes nearly unstoppable, as seen on Pagan. The Time Lord warned that the same can happen on Britannia. While it's known that he uses different worlds for different purposes, the ultimate reason for his world conquering is unknown.

The Guardian's magic is heavily based on geometric forms, and Blackrock, shown with his generators and the Blackrock Dome, which have a smaller copy within themselves. However, as Nystul explains, he is a sloppy spellcaster, casting with brute force. His other powers are many: observed are that he can kill people on the spot (if there is a connection to the Void), as he did with Batlin, can change his body size, and has extremely powerful attack spells, as he destroyed the great temple on Pagan with one lightning bolt in seconds, and then devastated the world. Also, he can cause things like ice ages, as done on Anodunos.

History
Before the Guardian directed his efforts to conquering Britannia, he conquered a number of other worlds, like Atarka and Talorus, and devastated others, like Anodunos and Pagan. A little over 20 years before Ultima VII, he decided to conquer Britannia, so he used his lackeys, primary Batlin, to create the Fellowship and divide the population, while three generators would weaken the land. He himself, would enter through a Black Moongate at the right time, and take over. To his anger, the Avatar foiled his plan at the last second, thus making him determined to conquer Britannia. His second plan came in motion in Ultima Underworld II, one year later. Enclosing the Castle Britannia into a Blackrock Dome, he planned for his troops from Atarka to invade from within under his champion Mors Gotha. But this plan failed as well thanks to the Avatar, who destroyed Dome and Portal to Atarka, angering the Guardian even more.

The Guardian had plans on the Serpent Isle in Ultima VII/2, but Batlin had his own. Angered by the treachery, the Guardian allowed Batlin to be killed. And as soon as the Avatar had repaired the damage done, the Guardian captured him, and exiled him to Pagan. The Guardian taunted the Avatar numerous times...and when the Avatar returned to Britannia, it seemed the Guardian had already conquered the land...

Trivia

 * In all installments, Bill Johnson is the famous voice of the Guardian.
 * In Ultima IX, we are told the Guardian is the Avatar's 'evil half', which was supposedly shed at the end of Ultima IV. Ultima fans ignore this, as it completely destroys the concept of what the Avatar was meant to be throughout the entire series, making it a mockery.