The Amulet and Dungeon Destard
Remember Sir Simon of Bordermarch? He gave you useful information about Lord British's Crown and Sceptre, but for the Amulet he relayed you to his wife, Lady Tessa. She simply tells you that it lies near the graves of valiant warriors: that means the knights that left with Lord British on the expedition. In the journey log all the details of the expedition are outlined and it can be followed to get the amulet starting from the waterfall in Spiritwood, East of Skara Brae.
In order to come back to the surface, though, you need to find your way through Dungeon Destard. Go to Jelhom and talk to Thorne and Trian: they will relay you to Goeth, who speaks backwards, but if you choose the right words he'll reveal the Word of Power to open Dungeon Destard.
Dungeons Shame and Doom
Remember Saduj? He was looking for an item inside a sandalwood box somewhere in Lord British's private chambers. But where was that? The solution lies far away. Tim of Empath Abbey told you about a song with magic powers, if played on an instrument worthy of a king, that is the harpsicord (old piano) in the king's room. Tim also talked about Kenneth, who resides in the lighthouse south of Trinsic. Kenneth will ask you to study the piece for "Stones", and will ask you some of the notes: answer "DCB" and "987", and he'll tell you all the notes to play on the harpsicord in Lord British's room atop his castle.
Back to Empath Abbey, Barbra told you her vision, that although not clear includes a vital clue to free Lord British in the depth of the Underworld.
There is only one dungeon left: Shame. The word of power is known by Sindar of Trinsic: talk to Woolfe about his son Jimmy to get a clue about the member of the Council. Under dungeon Shame lies the entrance of dungeon Doom, and its word of power is revealed by the Codex once the eight shrines of virtue have been visited.