Great Stygian Abyss

The Great Stygian Abyss is a massive volcanic dungeon, situated on the southern edge of the Isle of the Avatar. Long regarded as a haven for evil and it's minions, this dark place has long been considered to stand in opposition to the totality of the Eight Virtues. It features in Ultima IV, Ultima Underworld and Ultima IX as well as in the Runes of Virtue series.

Ultima IV
The Abyss was the ultimate goal of pilgrims seeking to complete the Quest of the Avatar in the early Age of Enlightenment, as within it's lowest depths lay enshrined the sacred Codex of Ultimate Wisdom. A place of desperate peril, the Abyss could only be conquered by those already having attain partial Avatarhood in the ways of the virtues, and only the Mystic Arms and Armour would serve to protect those who dared venture into its bowels.

During their journeys in Ultima IV, the Stranger would eventually manage to descend through the eight floors of this great dungeon, after fighting their way across the Bay of Pirates and unsealing the mouth of the Abyss using the Book of Truth, the Candle of Love and the Bell of Courage. After passing through numerous trials that tested their knowledge of the virtues, the hero would reach the Shrine of the Codex, and would attain the mantle of Avatarhood.

It was also at the mouth of this fell dungeon that the Stranger could endeavor to destroy the Skull of Mondain, by casting it into the flames of the Abyss. Later ages, however, would see this dark artifact resurface in Britain's Royal Museum, during the Age of Armageddon.

Ultima Underworld
While the Abyss would apparently lie dormant for years following the Great Council's effort to wrest the Codex from its depths, in the years following the Gargoyle Wars, a visionary knight known as Sir Cabirus would take interest in the feared dungeon, seeking to establish within it a colony to stand testament to the triumph of virtue over evil.

For a time, this bold experiment would yield favorable results, and numerous groups of virtuous goblins, mountain folk, men, trolls, and lizardmen lived in harmony within the caverns of what had once been Britannia's most dangerous locale. Sadly however, following Cabirus' untimely death, his followers would turn to bitter feuding, and the nearby Baron Almric would eventually be forced to seal the mouth of the Abyss as the surface world grew to see the failed settlement as little more than a breeding ground for monsters and wickedness.

It was to these sad halls that the Avatar would be forced to sojourn in the course of Ultima Underworld I, when the hero stood accused of conspiring to kidnap Almric's daughter, Arial, and was thereafter imprisoned within the Abyss under orders to secure her return from its depths. Eventually, the hero would succeed in rescuing the girl, and would furthermore stop the evil wizard Tyball from summoning a potent daemon known as the Slasher of Veils into the realm. To accomplish this, however, the Avatar would be forced to trigger an eruption of the abyssal volcano, destroying all that remained of the colony. The remaining members of Cabirus' company, however, were transported to safety by the spirit Garamon, and would later found a new settlement within the depths of the Dungeon Destard.

Ultima IX
The Abyss would also come to feature in the Avatar's last journeys in the realm, and would be the last dungeon the hero traversed in their quest to free Britannia from the Guardian's influence. In order to penetrate the mouthless caverns of the Abyss this time, the hero would be forced to call upon the powers of the fiery Titan of Pagan, Pyros, that they might descend into the earth. Here, the Avatar would pass through trials pertaining to elemental water, fire, earth and air before they would at last come upon the returned Lord Blackthorn, only to watch him be slain at the hands of Lord British in the midst of a magical duel.

Trivia

 * Although unvisited in Ultima III the Abyss is mentioned in the introduction of The Book of Amber Runes: "Surviving the spiritual transfiguration of the Great Stygian Abyss hath earned for thee the Mantle of Adept of the Sixth Circle. Few indeed have the strength or skill to cross the Abyss."
 * The Book, Bell and Candle needed to open the Abyss are traditional symbolic devices used in the ceremony to seal an excommunication in the medieval Roman Catholic Church. After condemning an excommunicant, a bell would be rung as a symbolic death toll; a book would be closed to signify separation from the church; and a candle would be snuffed to represent the extinguishing of the condemned individual's soul. Other videogames, such as Zork and NetHack also make use of these three items in order to access evil or hellish places.
 * In Runes of Virtue, the hero must descend into the Great Stygian Abyss to recover the Rune of Humility. The Great Stygian Abyss is also where the Black Knight hid the Mystic Armour, which he had obtained through nefarious means, and the hero could recover this fabled armour.
 * In Runes of Virtue II, the dastardly Black Knight makes his home on the level of the Abyss, where he imprisoned Lord Antonio of New Magincia. The hero must descend to the bottom of the dungeon and best the despicable knight.
 * In early drafts of Ultima IX, the Abyss was accessed deep beneath the ocean near the Isle of the Avatar, via a diving bell. Teeming with daemons and riddled with lava lakes, boiling mud, and ancient ruins, the vast complex of hellish caverns housed an iron keep as well as one of the Guardian's columns.