Exodus is the main villain from Ultima III: Exodus.
Exodus is a half demon, half machine hybrid created by both Mondain and Minax during the time when the two studied the dark arts together, being treated as if it was their son. While it's exact nature is a mystery, it is known Exodus is comprised of two distinctive components: the Psyche, which is more or less his soul, and the Dark Core, which is his brains.
Laying dormant for ages, Exodus awoke only to find that both of his "parents" had been killed by the Stranger. Seeking to avenge their deaths, Exodus patiently explored the world to accumulate information, slowly acquiring knowledge about more or less everything until the time was right. After much time of preparation, Exodus raised the Isle of Fire from beneath the seas and made it his base of operations, turning the Great Earth Serpent into his enforcer and guardian of his fortress, after which he summoned various monsters to plague the lands of Sosaria much like his creators did before.
Lord British once again summons the Stranger to Sosaria to help fight this new threat. The Stranger gather a party of companions to help him on his new quest, eventually finding his way into the Isle of Fire and Exodus's stronghold. The Stranger is unable to confront Exodus directly, however, as he find out about his true nature as a machine, so he uses four cards representing Sol, Moons, Love and Death, inputting them on Exodus core, which confounds the demon causing his Psyche to break away from the Dark Core, thus deactivating him.
With the ultimate defeat of Exodus, Sosaria finally finds peace, entering a new age of enlightenment as the lands are united under Lord British rule and renamed to Britannia. The Stranger would then undertake a quest to become a paragon to inspire people to lead virtuous lives, being known from there on as the Avatar.
During Ultima VI, the Gargish race manages to retrieve Exodus's Psyche from the void, placing it into their Shrine of Diligence, as they believed his diligence in learning about everything made him a perfect personification of the principle.