Doom

The caves of Dungeon Doom lie at the very center of the Underworld, surrounded by eternal darkness and rivers of molten lava. Rumors say that they lead to the center of the world. The cave dungeon of Doom is home to some of Britannia's fiercest creatures: dragons, sand traps, daemons and others. It is a difficult maze, where the seemingly easiest way forward is rarely the correct path to take.

Ultima V
It was at the very heart of the dungeon that the Shadowlords had imprisoned Lord British, trapped inside a magical mirror on the lowest level. The entrance to the dungeon was veiled by magical darkness that only Lord British's Amulet could pierce, and force fields sealed off the many passages in the labyrinth itself, impenetrable to everything but the magic of the Sceptre of Lord British. In addition to these extensive security measures, the Shadowlords themselves would manifest and engage any intruders who managed to find and enter the sealed dungeon.

The Avatar and the Warriors of Destiny located and unsealed the dungeon before descending into its depths. After successfully navigating the labyrinth and emerging triumphant from many bloody battles, they reached the center of the world. Only to be trapped inside the same magic mirror as Lord British. Fortunately, the Avatar had brought the sovereign's Sandalwood Box, which contained an Orb of the Moons that enabled their escape to the surface.

The escape from the mirror caused a cataclysm in the Underworld, destroying Dungeon Doom and collapsing the vast cave system. This caused further upheaval in the Realm of the Gargoyles as dimensional rifts into the Ethereal Void begun tearing the world apart, thus fulfilling the second sign of the Book of Prophecies.

Trivia

 * Although it uses the same game mechanics as the Words of Power for the other dungeons, VERAMOCOR (the word required to unseal Dungeon Doom) is technically not a Word of Power and consequently never actually referred to as such. In Ultima V, the Codex itself points the Avatar to Doom and commands the hero to call forth VERAMOCOR. The Great Council cannot possibly have sealed Doom like the other dungeons in Britannia as it lay beyond their reach, which means that the council did not create the "Word of Power" to unseal this dungeon. In fact, VERAMOCOR is the Word of Passage that was required in Ultima IV to gain access to the Codex from the lowest level of the Great Stygian Abyss.
 * While mystic/magical arms were required in the Abyss in Ultima IV (or Exotics for Exodus's castle in Ultima III), Mystic weapons are apparently not needed in Doom. "Up" and "Down" spells are disabled, like in the Abyss. The "Rel Por" (blink) teleport spell cannot be used in any dungeon rooms. Invisibility is completely non-effective in Doom.
 * Although the name is quite generic, Doom might be a nod to the dungeon from Akalabeth, which was called "World of Doom," or even simply "Doom"; in-game graphics from Akalabeth show a sign reading, "Doom" pointing at the entrance. It may also be related to the Dungeon of Doom from Ultima III'.
 * In the cancelled Ultima Online 2 and the Technocrat War trilogy, Doom was to be included as a dungeon on the frigid northern waste of Glacia.