The caves of dungeonDoom lie at the very center of the Underworld, surrounded by eternal darkness and rivers of molten lava. They are said to 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.
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 then veiled by magical darkness that only Lord British's Amulet could pierce, and many passages in the labyrinth itself were sealed off by forcefields 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.
In Ultima V, 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 but were 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.