This is the short version of the History of Ultima Online.  The original games, Ultima 1-9 were played offline on your home computer.  These games and their lore were the original inspiration for the Ultima Online expansion.

Ultima I - The First Age of Darkness
       
The beginning of the First Age of Darkness is marked by the coming of a sorcerer named Mondain. The father of Mondain had refused to share his secret of immortality with his son, and their disputes ultimately led to the father?s death. Torn with anguish and no doubt by his fears of persecution, Mondain turned his dark powers against the kingdoms of Sosaria. In desperation, Lord British called forth a champion to rise to the defense of the realm. The hero who responded to his summons would many years later come to be known as the Avatar. It was through the actions of this Avatar that Mondain?s foul gem of power was shattered and Mondain himself did come to a very sad end indeed.

Ultima II - Revenge of the Enchantress
   
The triumph of the Avatar did not last long, for in slaying Mondain he brought the wrath of Minax down upon the land. Minax was the young lover of Mondain and a sorceress with magical powers even greater than Mondain’s. She had the power to command legions of foul creatures, and in her quest for vengeance over the death of her lover, she brought much misery to the people of Sosaria. Again the hero who would come to be known as the Avatar returned to Britannia in the first recorded use of the Moongates. The Avatar slew Minax’s minions and did eventually destroy her as well. While there have been speculations as to the motivations of the Avatar, there is insufficient evidence to show that the Avatar was driven to violence by jealousy over Mondain’s romantic involvement with Minax. That being said, such theories are hereby denounced and should not be given consideration.

Ultima III - Exodus
   
Ultima III   
No one in all of Sosaria, not even the Avatar, could have realized that by ending the lives of Mondain and Minax, the Avatar would be orphaning their only child. The name of this unusual child was Exodus, and he was neither machine nor human. Exodus rose from the bottom of the Great Ocean to carry out a campaign of revenge and destruction against the land of Sosaria. So terrible were the forces unleashed by Exodus that the hero whom we would come to know as the Avatar required the assistance of a mysterious being known as the Time Lord to thwart them. And thus it was that the Avatar did deal with Exodus in a similar manner as he had dealt with his mother and father. Since that time much speculation has been given to the potentially immeasurable good such a creature as Exodus could have brought the land had he been persuaded to become beneficent, but I wish to formally disagree with those who say the Avatar should have handled the situation differently.

Ultima IV - Quest of the Avatar
   
Ultima IV   
After the smiting of Exodus, the people of Sosaria, who lived in terror during the onslaught of these evil magical beings, did unite together as a measure of self-protection under the sovereign rule of Lord British. Thus was formed the kingdom of Britannia. Led by wise Lord British, the land did come to flourish. The eight major townships rose upon the foundations of the old city-states. A renaissance of culture and civilization was highlighted by the formation of great institutions devoted to the study and advancement of the arts and sciences. It was at this time of growth and prosperity in the kingdom of Britannia that Lord British put out a call for one to show the way of spiritual growth and virtue. The call was answered by the hero who would come to be known as the Avatar. It was at this time that the champion of Britannia did come to earn the title of ‘Avatar’ by establishing the Eight Virtues and by seizing the Codex of Ultimate Wisdom from the depths of the Stygian Abyss. It has been said that the primary motivation of the Avatar has been personal redemption for the fate of Mondain, Minax and Exodus. While there has been no proof of this, I say that even if it were true, all the positive things brought forth in the world through the virtues would be enough to make amends for almost any misdoing.

Ultima V - Warriors of Destiny
   
Ultima V   
It seems that by removing the Codex of Ultimate Wisdom the Avatar had inadvertently set into motion a cosmic chain of events which led to the release of three Shadowlords from remnant shards of the black gem of Mondain. These Shadowlords were sinister agents of evil. Soon they managed to dethrone Lord British and hold him prisoner in a foul dungeon. Denied his wisdom and compassion, the kingdom of Britannia quickly grew oppressive and cruel. This was the Britannia that greeted the return of the Avatar. After a score of valiant struggles, Lord British was liberated, his throne regained and the Shadowlords banished from our world. However, Lord British’s escape from the underworld did cause a tremendously destructive series of earthquakes as the vast network of subterranean caverns collapsed. Much of the gargoyle race did perish in that tragic cataclysm.

Ultima VI - The False Prophet
   
Ultima VI   After the tremors that shook Britannia subsided, gargoyles, inhabitants of the other side of the world who now found their homeland virtually destroyed, began to appear on the Britannian side of the world in increasing numbers as aggressors. They launched vicious attacks against the human race and many were they who lost their lives in defense of our realm. The gargoyles even attempted to assassinate the Avatar. They set a Moongate trap which lured the Avatar into their clutches, but the Avatar was rescued by his companions. So it was that Lord British did once again call upon his champion to set the world right. Ending this violent racial conflict was the greatest challenge that the Avatar had ever faced. At last it was revealed that the Codex of Ultimate Wisdom, which had been removed from the underworld by the Avatar, was actually the property of the gar-goyles. Therefore, the Avatar placed the Codex into the great ethereal void where it would be possessed by neither human nor gargoyle. Two lenses, used to view the Codex, were given to Lord British and gargoyle ruler King Draxinusom. As the cause of the conflict between the two races had been removed, it was hoped that the rift between the two would eventually be closed. Those who would say that this terrible and destructive war could have been prevented entirely had the Avatar not appropriated the Codex from its true owners are merely dissidents who are grossly misinformed.

Ultima VII - The Black Gate
       
After many adventures throughout the land, the Avatar disappeared from the land, returning nearly two centuries later! This time, however, the hero appeared without summons, for no one knew of any cataclysms that required reparations. Yet it was not long before the Avatar was again facing great evil, this time in the form of an insidious organization called The Fellowship that had insinuated itself into Britannian society. In addition, a magical island, once home to the very Exodus of the Third Age of Darkness, had risen to the surface, indicating a further threat to Britannia. During this seventh period of service, the Avatar was able to uncover the dastardly plot of The Fellowship and remove the evil presence forming on the island.

Ultima VII Part II - The Serpent Isle
   
Ultima VII Part 2   
After many adventures throughout Britannia, the Avatar disappeared from the land, returning nearly two centuries later! This time, however, the hero appeared without summons, for no one knew of any cataclysms that required reparations. Yet it was not long before the Avatar was again facing great evil, this time in the form of an insidious organization called The Fellowship that had insinuated itself into Britannian society. In addition, a magical island, once home to the very Exodus of the Third Age of Darkness, had risen to the surface, indicating a further threat to Britannia. During this seventh period of service, the Avatar was able to uncover the dastardly plot of The Fellowship and remove the evil presence forming on the island.

Ultima VIII - Pagan
   
Ultima VIII   
Banished to an unfamiliar land by the Guardian, the Avatar was forced to find a way out of the world known as Pagan. Whilst upon this land, the Avatar discovered several new forms of the arcane outside the Eight Circles of Magic. They would prove useful in escaping, but would it be in time to save his beloved Britannia?

Ultima - Ascension
       
The last of the guardian saga. For the final time you have been called to Britannia to restore the Virtues and bring peace to the land. The guardian has brought his wrath upon Britannia and is close to completing his conquest. Old friends return to assist the Avatar to defeat the guardian for all time...

History of Sosaria
http://uoevolution.com/wiki/index.php/History_of_Sosaria#Ancient_Sosaria

UO Timeline of Events
http://uoevolution.com/wiki/index.php/History_of_Sosaria#TIMELINE