The Guardian first appeared in Ultima 7 as an extremely powerful being that threatened to conquer Britannia should he manage to enter it. As a big, red "extradimensional being", as the wisps described him in The Black Gate, his appearance was similar to that of a demon, and this made him look very evil. One very nice feature that started in The Black Gate was the Guardian's voice, excellently interpreted by Bill Johnson, which accompanied you in your travels during your quest to destroy the Black Gate.
The Guardian's plan was initially to enter Britannia by means of the Black Gate, a dimensional gate made of Blackrock, which would function as a portal between worlds during the Astronomical Alignment, which took place every 800 Britannian years. To build this Black Gate, he first had Batlin found the Fellowship, a double-faced organisation that promoted brotherhood, unity and worthiness, yet which performed criminal acts (including murder) behind the scene. The Fellowship eventually managed to build the Black Gate, but although the members of the inner circle died to defend it, the Avatar managed to destroy the Black Gate using Rudyom's wand just as the Guardian was about to enter Britannia. Batlin, the founder of the Fellowship and the Guardian's right hand, managed to escape via teleportation.
One year after the destruction of the Black Gate, the Guardian put another plan in practice, and this is the story we see in Ultima Underworld 2. Since I cannot run this game, Crowley has been kind enough to fill me in on the story:
While several important people were attending a festival at Castle British one year after the destruction of the Black Gate, Guardian created a huge blackrock dome around the castle with the help of an unknown agent within the castle. The Avatar descended to explore the caverns beneath the castle, and found a large blackrock gem within (as Nelson notes in the game, this was not unlike the three prisms within the three generators). This blackrock gem provided a gateway to other dimensions. According to Nystul, this was because the Guardian's spell to create and sustain the dome was drawing power from many planes he had conquered. There were eight other planes you could visit.
To bring down the blackrock dome the Avatar had to collect small blackrock gems from each of the worlds, which served as a magical support structure of sorts for the large dome. He also had to block sources of power from which energy was drawn to support the spell. While Avatar was accomplishing this, Guardian sent dreams and visions to him and others trapped in the castle of how his armies were marching across Britannia. He also tried to convert many to work for him by promises or threats. Mayor Patterson succumbed, and killed Nelson and Lady Tory in the castle before he was uncovered and killed.
As the Avatar was near to shattering the blackrock dome, Guardian sent his forces through the gem to Castle Britannia, led by Mors Gotha, who was the Guardian's champion in a similar way as Avatar was Britannia's. In the end, Avatar slew Mors Gotha as she screamed for Guardian to save her. There was no answer.
the final blow came from the Horn of Praecor Loth. Avatar him/herself lacked the strength of lungs that was required to blow it, so (s)he bound an air demon (djinn) to him/herself, instructed by Zoranthus. Also, the lines of power were cut by a magic wand Altara made especially for this purpose. She told the Avatar to look for a place in each world where the Guardian had done some influental act. In Prison Tower, this was a force field around Bishop's cell, in Killorn Keep the room with the brain creatures, in Anodunos the frozen fountain in the middle of the city, and so on.
The next chapter in the Guardian's history took place on Serpent Isle, where Batlin had escaped. The Avatar was sent to Serpent Isle to find Batlin after a scroll from the Guardian had been found among Batlin's belongings. When the Avatar eventually found Batlin, three Banes of Chaos escaped him, and the Guardian killed him. These Banes of Chaos were the result of an Imbalance that threatened to destroy Serpent Isle and all other worlds in the universe, and they proceeded to annihilate nearly the entire population of Serpent Isle, with a few lucky survivors. However, the Avatar still managed to restore Balance, thwarting the Guardian's plans and earning the title of Great Hierophant of Balance.
However, when restoring Balance, the Avatar was teleported into the Ethereal Void, thus being exposed to the Guardian. Mocking him, the Guardian did not hesitate to grab the Avatar and hurl him onto another world which he had already conquered - Pagan. While the Avatar was occupied on this world where the Guardian was known to be a protector, when in reality he was both the Guardian that had warned them of the coming of a Destroyer and the Destroyer himself, the Guardian managed to enter Britannia. When the Avatar managed to defeat the Titans that ruled Pagan with an iron fist and create a Black Gate with their powers to get back to Britannia, the first thing he saw was a monument to the Guardian. What exactly this was supposed to be is not known for sure, but the developers of Ultima 9 interpreted it as the Guardian's keep at Terfin.
The story of Ultima 9 is not very accepted among Ultima fans. This game sheds light on the Guardian's origin, and brings the great villain to an end. According to this game, the Guardian is the Avatar's "evil twin", which was separated from the Avatar earlier. To thwart the Guardian, the Avatar has to perform a ritual to fuse with him and ascend into the sky to form the constellation of the Ankh. After this, the Guardian and the Avatar are one being. In Ultima X, they will supposedly be one thing fighting for control over itself, and other people achieving Avatarhood can help the Avatar win the struggle.
The Guardian is known most for his thirst for power and his desire to conquer new worlds, especially Britannia. The reasons for his desire for conquest are unknown, though some things seem to hint at his need for more resources to use in further conquests. Britannia, being rich in minerals, forests, wildlife and other resources, may have been an ideal target.
Crowley explains: "It would seem that the Guardian applied varied methods to take over various dimensions. He struck deals, offered help or orchestrated an all-out interdimensional war or entered the plane himself if he deemed that necessary. A common nominator would seem to be that if Guardian's servants try to turn against him, response will be swift, but not necessarily a large show of force. Apparently a world may live peacefully with its people seemingly content for centuries under the Guardian, as long as they don't try to stage an uprising. Zoranthus explained that various planes provide various goods for the Guardian's conquest. The world where Pits of Carnage was located supplied many vicious and loyal warriors, whereas for example runestones were produced in large quantities for Guardian in Talorus (and the Runekeeper there did not question producing them, since their application was not in his(?) function)."
However, the Guardian's conquest appears to have gone far beyond his control. At one point, when too much land is conquered, the conquerer cannot cope with it and eventually finds that he no longer has sufficient resources to control all of it. In real history, the Roman Empire fell this way, and the Guardian might also have faced a similar defeat if Ultima 9 was made with the right spirit.
Crowley says that: "In the Pits [of Carnage] resided a mage named Zoranthus who also told Avatar about the multitude of planes and the Guardian's lust for them. He said that while Guardian's resources were huge, they were finite as it seemed that he had drawn forces from too many planes to be stationed elsewhere."
The Guardian commonly spread his influence by speaking to the people of a world he wanted to conquer, or one which was already under his power. He influenced Britannians by speaking to them as an "inner voice". However, this required the Cube Generator, and it is not known how he manages to speak with people in Britannia or other worlds in the absence of such a generator.
Crowley adds: "It seems that it is a rather common practice for Guardian to speak directly into the minds of those he deems particularily useful for his cause, suggesting actions and promising rewards (as seen with the inner voice of the Fellowship and voice of Brom for Gorn). In the world of Killorn Keep hearing the voice of the Guardian was generally considered a great honor."
As a being of pure evil that appeared out of nothing in Ultima 7: The Black Gate, the Guardian has always been a mysterious character. The question we usually ask ourselves is "What is the Guardian?" The official explanation, that surfaces in Ultima 9, is that the Guardian is your "evil twin". However, most fans don't accept that, as it's a pretty lame explanation for a character that is supposed to be the embodiment of evil and which is far more powerful than anyone on Britannia.
There are many other theories about the Guardian's origins, and many of them have been discussed on this topic on the Exult forum. Those I've heard of, some mentioned in that topic and others not, include:
INTRO HISSS JIVE, or by pressing F11 in Exult) where the Guardian is a big red muppet
An interesting thing about the Guardian is that his teeth change more than Castle British! Some people wonder who his dentist is, considering his teeth change from a lump of crystals to a perfect set of teeth. See for yourself:
First from left: The Guardian in The Black Gate intro - red, crystal-shaped teeth
Second from left: The Guardian in The Black Gate endgame - white, wide human-like teeth
Third from left: The Guardian in the Serpent Isle intro - yellow, crystal-shaped teeth
Fourth from left: The Guardian in the Ultima 9 intro - white, thin human-like teeth
Fifth from left: The Guardian/Avatar hybrid in the Ultima X trailer lives up to the Guardian's tradition and comes with a totally new set of dent-- er, teeth - sharp, white teeth
Nothing can explain why the Guardian's teeth keep changing shape and colour. However, a few people have come up with some interesting theories (which they submitted at this thread in the Exult forum). Here are the theories:
His teeth are red in BG and Yellow in SI because he doesn't brush his teeth, and they show color from food stains from his last meal. They rot and his dentist had to remove them all and put some dental prothesis."
Maybe the Guardian has/uses a double/standin like in the movies or maybe he discovered the secret of acne medication from SI which he uses for his teeth."