Hythloth is the dungeon of
Humility in
Britannia in the
Ultima series of games, although it can be argued it is the dungeon of
Spirituality in some games, too. Unlike the most of the other dungeons, Hythloth is a meaningless word, not an anti-virtue like
Shame or
Covetous. In most of the Ultima games, the entrance is consistanly located on the
Isle of the Avatar, in the south-east corner of
Britannia.
Since Ultimas I, II, and III don't take place in Britannia, there is no Hythloth there.
In Ultima IV, Hythloth can be accessed through the regular dungeon entrance on the Isle of the the Avatar, and through a hidden entrance in the castle of Lord British. The white stone is missing is missing from Hythloth and can be found elsewhere, but Hythloth houses powerful orbs that increase all of a characters attributes, and the balloon is located at the exit.
In Ultima V, the entrance has been sealed with the word of power "IGNAVUS". The Shard of Cowardice and mystic weapons and armor can be found in the underworld near the exit to Hythloth.
In Ultima VI, Hythloth connects Britannia with the Gargoyle world. In addition, Captain Johne, who knows how to communicate with the gargoyles, lives here.
Hythloth is strangely absent from Ultima VII, and since Ultima VIII doesn't take place in Britannia, it doesn't appear in that game either.
And finally, in Ultima IX, Hythloth appears north of New Magincia, instead of on the Isle of the Avatar, and it is mostly submerged in water. The backstory was updated, turning Hythloth into the sewers of Old Magincia, now abandoned and full of foul poisons. The Glyph of Humility is located here.