Exodus
Exodus was a villain, the spawn of Mondain and Minax.
He was based out of Ilshenar, in a dungeon (aptly called Exodus Dungeon) in the central region of that facet. His meddling in the timeline of Ilshenar caused the official fiction of that facet to change. He was behind the invasions of certain cities by the Juka, together with Lord Blackthorn. Indeed, it is Exodus who is credited with "assisting" Blackthorn's transformation from philosopher of Chaos and spoiled, embittered nobleman to full-fledged evil.
Exodus was never seen in-game in UO, but it was said in February 2003 by Dawn, a Royal Knight of the realm, that Exodus had greatly faded and that his power was waning in Ilshenar, following the death of Blackthorn. (Apologies for earlier misreporting that Dawn had stated that Exodus had died.)
Ultima lore, from the single-player games, holds that Exodus was the prodigy of Mondain and Minax, and alludes to "it" being a sentient computer of some kind- programmed by the use of four different punch cards. The highly mechanized features of the Exodus dungeon appear to back that up to some degree.