Hythloth

Hythloth [hith-loth], the hell-hole, is a dungeon located on the southern shores of the Isle of the Avatar, near the Shrine of the Codex. One of the eight famed Britannian dungeons, it was first explored by the Stranger during the Quest of the Avatar and would be returned to in subsequent adventures.

Ultima IV
The anti-Spirituality Dungeon of Hythloth was first entered in Ultima IV. The White Stone was missing from it, but the magic orb in it gave the best experience boosts and it connected to all three Altar Rooms. Outside of Hythloth, the Balloon was found. After the end of the Quest of the Avatar, the Great Council sealed the dungeon, to lock in the horrors within.

Curiously, during this period Hythloth had a secondary entrance just behind the castle of Lord British, one which opened up directly to the lowest and most dangerous level of this hellish place. The reasons for this have never been explained. This entrance was removed when the Great Council sealed the dungeon, and was never reopened.

For a detailed solution, see: Solution for Hythloth in Ultima IV

Ultima V
The mine started to decay in Ultima V, when Hythloth again was visited. The Avatar yelled the Word of Power "IGNAVUS", learned from Hassad, to open the entrance. It connected to the Underworld, where the Shard of Cowardice and the Mystic Weapons and Mystic Armour were found.

For a detailed solution, see: Solution for Hythloth in Ultima V

Ultima VI
After that time, Hythloth fully became a cave system again. In Ultima VI, it was the only connection to the Gargoyle Lands and Captain Johne lived in it.

For a detailed solution, see: Solution for Hythloth in Ultima VI

Ultima VII
After that time, the settlers under Sir Cabirus re-sealed the dungeon, so that the nasties in it won't threaten the colony.

Hythloth wasn't called by name in Ultima VII, but it is logical that the dungeon present in that game be a part of Hythloth. The main entrance was still sealed in Ultima VII, but the Fellowship had set up camp in a different part of the dungeon.

For a detailed solution, see: Solution for Hythloth in Ultima VII

Ultima IX
In Ultima IX, Hythloth was mutated into a sewer in Ascension, moved to New Magincia, and was opposed to Humility. This contradicts some previously established facts from other versions of the game.

For a detailed solution, see: Solution for Hythloth in Ultima IX

Trivia

 * In the first appearance of Hythloth on the Apple II version of Ultima IV it is spelled "Hylothe". In every port of Ultima IV it was changed to Hythloth and the change was maintained in every subsequent Ultima.
 * Skara Brae is the city of Spirituality, the Virtue opposing Hythloth. This creates an apparent discrepancy in Ultima V, as the councilor Hassad, which had Hythloth's Word of Power, was kidnapped from New Magincia and not from Skara Brae. However, it is made clear that he actually represents Skara Brae in the Great Council, thus establishing consistency.
 * The reason why Hythloth in Ultima IX looks like a sewer is because this dungeon wasn't actually supposed to be Hythloth; instead, these were meant to be the sewers running under Britain, as seen in earlier games. However, when the plot of Ultima IX was altered, the sewers ended up being one of the many things cut from the game. Apparently not wanting the work to go to waste, Origin simply recycled the sewers' assets to create the new Hythloth.
 * An entire section of Hythloth in Ultima IX was deemed too difficult by play-testers and the dungeon was truncated. Curiously, however, that section was left in the finished game instead of being deleted or sealed off; there is only an optional transporter disc that allows the player to bypass the final areas.