Guardian

The Guardian is the main adversary of The Avatar during the Age of Armageddon. The Xorinite Wisp describes the Guardian as a malevolent, egocentric being from another dimension, who thrives on domination and control. The Guardian has demonstrated 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 shows extreme resistance, he destroys it, as he did with Anodunos. If the Guardian enters a world outside of his own dimension, he becomes nearly unstoppable, as seen on Pagan. The Time Lord warned that the same is true in Britannia. While it's known that he uses different worlds for various 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.

A few abilities the Guardian has demonstrated:
 * Killing people on the spot (if there is a connection to the Void), as he did with Batlin
 * Changing his body size
 * Extremely powerful attack spells (he destroyed the great temple on Pagan with a single lightning bolt, then devastated the entire world)
 * The apparent ability to change whole climates (he brought about an ice age on Anodunos).

One important aspect of the Guardian is that he 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 towards conquering Britannia, he conquered a number of other worlds, like Killorn Keep's world and Talorus, and devastated others, like Anodunos and Pagan. A little over 20 years before Ultima VII, he decided to conquer Britannia, using his lackeys (primarily Batlin) to create the Fellowship and divide the population, while three generators simultaneously weakened the land. When all was ready, the Guardian would enter Britannia himself through a Black Moongate and seize control. To his anger, the Avatar foiled his plan at the last second, thus making him determined to conquer Britannia. His second plan came into motion a year later, in Ultima Underworld II. Enclosing the Castle Britannia inside a Blackrock Dome, he planned for his troops from Killorn Keep to invade from within, led by his champion Mors Gotha. This plan failed as well, thanks to the Avatar, who destroyed the Dome and Portal to Killorn Keep after slaying Mors Gotha in the process.

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

The Guardian had 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 fused with the energy of the Avatar, through special use of the Armageddon spell.

During the events of Ascension, the Avatar learns from Shamino that the Guardian was created from the Avatar's "evil half," which was supposedly shed at the end of Ultima IV. It should be noted that many Ultima fans entirely reject this notion, as they deem it to be completely inconsistent with the concept of what the Avatar was meant to be throughout the entire series, and 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 is more consistent with Ultima lore, and also seems to explain 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 VIII was released.