The Isle of the Avatar was discovered during the Fourth Age (as related in Quest of the Avatar), at the climax of the Avatar’s quest. The Stranger from Another World, who came to Britannia in answer to Lord British’s summons, completed the Test of the Eight Virtues and became the Avatar — the embodiment of the pure axiom of life. It was during his journeys that the Avatar unearthed a chilling artifact: the skull of Mondain the Wicked, the first of the Triad of Evil. There were those who claimed that the artifact held the power to destroy all life on our world. Fortunately, it was the Stranger who discovered it, and not someone of lesser Virtue. Upon completing the Path of the Eight Virtues, the Avatar embarked on a search for the Codex of Ultimate Wisdom. Legend linked this ancient tome with the subterranean maze known as the Great Stygian Abyss. “Truly, no one has ever plumbed its depths,” wrote Shamino, who arranged by magic to obtain a map of its corridors. “I cannot imagine the horrors that await the first to venture into the Stygian depths. ”

Setting Ultima Underworld is set in Britannia, the fantasy world of the Ultima series. Specifically, the game takes place inside a large, underground dungeon called the Great Stygian Abyss. The dungeon's entrance lies on the Isle of the Avatar, an island ruled by Baron Almric. The Abyss first appeared in Ultima IV: Quest of the Avatar, in which it contains the player's final goal, the Codex of Ultimate Wisdom. Ultima Underworld is set after the events of Ultima VI: The False Prophet; in the time between the two games, a man named Cabirus attempted to create a utopian colony inside the Abyss. The eight settlements of the Ultima series each embody one of eight virtues, and Cabirus wished to create a ninth that embodied all virtues. To achieve this, he united diverse cultures and races in peaceful co-existence and planned to promote harmony by giving each group one of eight virtue-imbued magical artifacts. However, he died before distributing the artifacts, and left no instructions for doing so. As a result, the colony collapsed into anarchy and war, and the artifacts were lost. At the time of Ultima Underworld, the Abyss contains the remnants of Cabirus's colony, inhabited by fractious groups of humans, goblins, trolls and others. Story Before the beginning of the game, the Abyss-dwelling wizard brothers Garamon and Tyball accidentally summon a demon, the Slasher of Veils, while experimenting with inter-dimensional travel. Garamon is used as bait to lure the demon into a room imbued with virtue. However, the demon offers Tyball great power if he betrays Garamon. Tyball agrees, but the betrayal fails; Garamon is killed, but seals the demon inside the room. Because he lacks virtue, Tyball cannot re-enter by himself, and plans to sacrifice Baron Almric's daughter, Arial, at the doorway to gain entrance. In the game's introduction, the ghost of Garamon haunts the Avatar's dreams with warnings of a great danger in Britannia. The Avatar allows Garamon to take him there, where he watches Tyball kidnap Arial. Tyball escapes, leaving the Avatar to be caught by the Baron's guards. The guards take him to the Baron, who banishes him to the Great Stygian Abyss to rescue Arial. After the introduction, the Avatar explores the dungeon and finds remnants of Cabirus's colony. A few possible scenarios include deciding the fate of two warring goblin tribes, learning a language, and playing an instrument to complete a quest. The Avatar eventually defeats Tyball and frees Arial.