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, megalomaniac 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 through 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.

One important aspect is, that the Guardian is very fond of personally torturing his victims psychologically with taunts, nightmares and dark visions. More about this here: Taunts of the Guardian.

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, primarily 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 after slaying Mors Gotha in the process, 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 of Britannia...

The Events of Ascension
The Guardian created eight columns to rip Britannia apart and cause the moons to crash on the planet, while the people were twisted in their ways. In the end, the Guardian was destroyed, when his energy fuses with the energy of the Avatar through special use of the Armageddon spell.

At the final confrontation with the Avatar, the Guardian reveals that he is the Avatar's 'evil half', which was supposedly shed at the end of Ultima IV. It has to be noted that most Ultima fans reject this notion, as it is completely inconsistent with the concept of what the Avatar was meant to be throughout the entire series, and because it also causes several other plot inconsistencies (one example being the storyline of Silver Seed, wherein it appears that the Guardian had existed long before the Stranger became the Avatar).

Trivia

 * In all installments, Bill Johnson is the famous voice of the Guardian. Interestingly Michael Dorn from Star Trek: The Next Generation had initially been hired to voice him in the original version of Ultima IX and reportedly recorded his lines, but the team eventually decided to bring Bill Johnson back as he was the true Guardian.
 * Guardian's origin began to change when the cuts were made to Ultima VIII:Pagan. All the subtle hints revealed from Ultima VII onwards hinted Guardian being the ruler of Pagan, also hinting Pagan being his homeworld. The released version of Ultima VIII shows no signs of the previous hints, hence throwing Guardian's "true" origin into mystery.
 * The unreleased add-on, The Lost Vale would have been the last chance to explain the origin of Guardian before Ultima IX.
 * Interestingly, Richard Garriott revealed in the Ultima IX Hint Books the actual origins he had written for the Guardian in Ultima IX, before it was inexplicably simplified in the final release. The Guardian was actually meant to be tied with the Shadowlords and the Gem of Immortality: Rather than having the Avatar's evil half simply "cast away" as he achieved Avatarhood, the Gem of Immortality was meant to resonate with the Avatar at the time he had originally destroyed it, thus spawning the Shadowlords -- the "Dark Side" of the Avatar. As they are banished in Ultima V, the Shadowlords coalesced in the Void to form a new entity calling himself the Guardian, who basically came back to take revenge upon the Avatar. (Note that this explanation while being more consistent with Ultima lore, also seems to explains many plot holes in the final game).
 * Also, in a past gaming convention, Richard Garriott had told a fan the original origin of Guardian was that he was part of a family of similar beings of power. This was before Ultima 8 was released.