Exodus is the main villain of Ultima III, the creation of the evil wizard Mondain and his young apprentice/lover Minax, who considered Exodus their "child". The three were collectively known as the Triad of Evil, the three main villains of the Age of Darkness.
Exodus is part daemon, part magical entity and part machine. The daemon part is Exodus' psyche, his personality and lifeforce, while the machine part stores his power and knowledge, contained in a hard drive-like device called the Dark Core. The "magical" part is probably what allows Exodus' daemonic and machine parts to work together.
Exodus waited a long time after the death of his creators, gathering information about the world and about his enemies. Exodus appears to have a great curiosity, wanting to learn about everything. Most of the information he collected was trivial, like the color of the sky on a particular day; however, he also was able to learn how to manipulate the very essence of the world itself.
In Ultima III Exodus attacked, poisoning the spirit of the land itself while his hordes roamed Sosaria. He raised the Isle of Fire from the bottom of the sea and turned it into his fortress, building the Castle of Fire. He protected the Isle with the Great Earth Serpent, which he had ripped out of the void and enslaved to his will (this abduction of the Serpent in turn caused the War of Imbalance on the Serpent Isle). Thus began the Third Age of Darkness in Sosaria.
Exodus' end came when the Stranger and three companions successfully invaded the Castle of Fire. They used the Four Cards to destroy or deactivate the machine part of Exodus, though the Dark Core itself survived. The destruction of the machine caused the daemon Exodus to vanish from the world.
Embodiment of Diligence
Even this wasn't the end of Exodus, however. His psyche was somehow raised by the gargoyles, who embodied it in their Shrine of Diligence. But this act destabilized the Isle of Fire, causing it to sink back beneath the sea.
The Avatar met Exodus -- now united with the Shrine of Diligence -- in Ultima VI. Exodus explained that diligence had been the essence of his soul; cold, unswerving diligence in his path of conquest. But in his devotion to achievement he lost sight of his original goals. Diligence without a goal is as worthless as a goal without diligence. The means cannot replace the end, and diligence cannot itself be the goal. He saw that the gargoyle society was based on the diligent leading the wayward. In that way they maintain their fight for survival in their harsh world.
Return of the Isle of Fire
His leftover machine portion, the Dark Core, reappeared in Ultima VII after the Isle of Fire surfaced once again from the seas. The Avatar managed to cast the Core into the Void, preventing it from reuniting with Exodus' psyche -- an event which may very well have resurrected Exodus, had it occurred.
- The Four Cards needed to destroy Exodus are computer punch cards, showing Exodus' technological origins.
- In the Ultima Online 2 background story, Exodus was a remnant of an Overlord technology from the future of Britannia which had been sent back in time following the magical Cataclysm that create New Britannia in the first place.