Ultima VI: The False Prophet/Dungeon Hythloth

News about the Underworld
As Mariah suggested, travel to the Dry Lands (near the Shrine of Sacrifice, east of Minoc) and ask Sin'Vraal about the Book of Prophecies and about sacrifice.

There is more about dungeon Hythloth. Many characters, especially from Buccaneer's Den, link it to Captain Johne. The gossip about him are reported here, starting from the most wrong. Indeed, only Leodon and Zoltan have reliable information about this character.

One last note about one inhabitant of the underworld, Beh Lem, is referred by the wandering bard Mandrake:

Dungeon Hythloth
'"Hythloth" is a shortened form of "high sloth". According to the Ultima philosophy, Sloth is the opposite of Spirituality. As the virtue of Spirituality comes from the union of the three principles of Truth, Love and Courage, the sin of Sloth comes from the union of the three anti-principles of Falsehood, Hatred and Cowardice.'