Ultima Prime is a term used by some players for the Ultima single-player games upon which UO is based, and the fiction that underlies them.
In Ultima I, the Gem of Immortality - which contained a complete copy of Sosaria - was splintered into Shards in order to defeat Mondain. The original "Prime" world continued on its pre-ordained course (the single-player games), whereas life on the Shards continued along without the Stranger (who destroyed the gem).
Ultima II: Revenge of the Enchantress
Ultima III: Exodus
In Ultima III: Exodus, the Stranger returns to Sosaria to find the world much changed from the last visit- both in the amount of time passed, and the reformation of the contenents. This time, a new evil threatens the land: Exodus, the prodigy of Mondain and Minax's dark magic. Or, is Exodus a hellish combination of science and sorcery?
Ultima IV: Quest of the Avatar
In Ultima IV: Quest of the Avatar, the Stranger returns to Sosaria to find that with the destruction of Exodus, the world was once again devastated- islands rose and sank; fertile plains became desserts; and the formation of the continent was again drastically altered. Coming out of the Dark Ages of Mondain, Minax, and Exodus, Lord British is searching for a new moral code to unite his people. Becoming a paragon of the 8 Virtues, and their 3 Principles, the Stranger officially becomes known as the Avatar. Braving the darkest dungeons, and ripping the Codex of Ultimate Wisdom from the depths of the Abyss, the Avatar brings a new age of enlightenment to Lord British’s realm.
Ultima V: Warriors of Destiny
Ultima V: Warriors of Destiny finds Britannia in crisis! Lord British is missing, and Lord Blackthorn is acting as regent king. Only, something is amiss with the new regency. Blackthorn has converted the original Virtues, and now they offer something darker, something more self-serving. The Avatar discovers that when the Codex was ripped from the depths of the Abyss, immense underground caverns were created, thus exposing three shards from the corrupt Gem of Immortality. These shards eventually possessed three people, and transformed them into the Shadowlords—the maligned influence behind Lord Blackthorn’s rule and Lord British’s kidnapping.
Ultima VI: The False Prophet
Ultima VI: The False Prophet finds the Avatar in mortal peril. Typically, when the Avatar traveled across the Ethereal Void void, it was through a Blue moongate—this time, when the Avatar is pulled to Britannia, it is through a Red moongate. Not only is the gate different, but the arrival locale is too! The Avatar was summoned to some sort of demonic sacrificial altar. Iolo, Shamino, & Dupre arrive in the nick of time, and rescue the Avatar- gating back to Lord British’s castle. The Avatar once again learns of massive destruction- when Lord British was rescued from the Underworld, massive earthquakes struck the land, and the subterranean catacombs collapsed upon themselves. Now, Britannia and its Shirnes of Virtue are being overrun by Gargoyles. It was they who tried to sacrifice the Avatar! Unbeknownst to Lord British and the Avatar, the Codex of Ultimate Wisdom used to be the Gargoyle’s sacred text- which was stolen from the Abyss; and, when the subterranean catacombs collapsed- they collapsed on top of the Gargoyle city states! The Avatar may have been a savior for the land of Britannia, but the Garolyes only see him as a False Prophet! And now, a time of reckoning has come.
Ultima VII Black Gate & Serpent Isle
Ultima VII: Black Gate & Serpent Isle opens with a murder. A ritualistic murder brings the Avatar back to Britannia. During the course of the investigation, the Fellowship religion is looked into, and the the driving force behind the Fellowship is the Guardian. The Guardian talks to its followers, helps its followers, empowers its followers, and the Guardian wants its followers to build a device out of Blackrock that will generate a Black Moongate. How could this possibly be a bad thing? The Avatar must expose the Fellowship for what they are (and the Guardian), and pursue the ringleaders across Britannia, through the Serpent Pillars, and onto the Serpent Isle, the ancesteral home of the Ophidian Race.
Ultima VIII: Pagan
Ultima VIII: Pagan picks up where Serpent Isle left off - with the Avatar racing through a questionable moongate, to pursue the Guardian's forces. In doing so, the Avatar finds himself in the alien land of Pagan, where the Guardian holds dominion, and nothing is pleasant. The Avatar must learn a new system of magic involving Pagan Reagents, and overcome elemental forces. To survive, to escape, choices of what is the lesser evil must be made.
Ultima IX: Ascension
Ultima IX: Ascension summons the Avatar back to Britannia one last time. The twin moons are out of alignment, on a collision course with Sosaria, the moongates are unstable, and magic is unpredictable at best. When the Avatar seeks assistance, he finds that his old comrades are either missing, insane (blackrock infection?) or have become corrupt from the Guardian's influence. The Avatar must finally acknowledge that he is both savior and destroyer - when he became the Avatar, his shadow self (the non-virtue traits of himself that he cast off) became personified, and became the force known as the Guardian.
Ascension ends with the restoration of the virtues, the realignment of the moons, and the Guardian reintegrating into the Avatar’s self. The Avatar/Guardian then ascends into the heavens and becomes a new constellation, the Ankh.