Hidden village of Cove
Romasco of Skara Brae started a thread about the Great Stygian Abyss. He sent the companions to Zair the Wise in Paws who, in turn, directed them to his brother Zair in "a village beyond Lock Lake".
Jingles of Paws spoke about the "Gate Travel" spell and directed the Companions to his master Mentorian in "a village beyond Lock Lake".
Zorin of Castle Britannia told the Companions to visit the three castles and speak to all named Antos. Then, if you go to the nearby Castle of Love (Empath Abbey), the people there direct you to "the village beyond Lock Lake", too.
Well then, how can we cross Lock Lake?
In the NES remake only, two inhabitants of Buccaneer's Den reveal the secret to reach Cove: the barkeeper (The Captain) and a pirate in the pub (J. J. Scirlock). Leave a 100 GP tip at the pub, and learn about the link between the whirlpool and Lock Lake.
The whirlpool moves randomly around the seas, but because of this randomness it generally remains in the same area. Apparently, it can be often found in the sea between Jhelom and the Shrine of Honor (e.g. Amiga and Master System ports), but it can also get past Cape of Heroes, south of Trinsic.
As mentioned above, in Cove you can ask Brother Zair about the Word of Passage. He will reveal that you have to talk to all the lords of the strongholds of Love, Truth and Courage. Also, Mentorian teaches the recipe for the "Gate Travel" spell.
Two characters in Castle Britannia, Joshua and Juliet, spoke about an "Axiom Riddle". More on this can be learned in Cove; four mages stand around a fire in the south-west woods: Spellbind, Shaman, Circe, Charm; they all give clues about the solution of the riddle.
More threads start in Cove: Allen talks about the entrance to the Abyss and a magical ship wheel. The companions are relayed to the seahorse Blissful and to the shrine Ankh to learn more.
Leave Cove, sail across the lake and leave the ship on the shore.
Empath Abbey, stronghold of Love
The nearest castle is Empath Abbey.
In the throne room, ask Lord Robert about the Word, and he will reveal one of the three syllables.
Find Brother Antos and ask him about the Candle of Love. The Stranger will be redirected to the Ankh of Life (in the shrine) and then to Derek the bard, who is beyond a double secret passage in the west wall of the throne room. He will direct the Companions back to Cove: there is a secret passage in the west wall of the shrine; search the floor to find the candle.
The Lycaeum, stronghold of Truth
The Lycaeum can be reached from the moon gate south of Trinsic. The moongate appears at the gibbous waning left-hand moon; its second destination (right-hand new moon) sends to Verity Isle. Travel north to the Lycaeum.
In the throne room, ask Lord Robert Frasier about the Word, and he will reveal one of the three syllables.
Find Father Antos and ask him about the Book of Truth. The Stranger will be redirected to the library. Jimmy the locked door, and ask the librarian Lord Terence about the book you search. Search between R and V for the Book of Truth.
Serpent's Hold, stronghold of Courage
Serpent's Hold is on an island without any Moon Gate. The island is located to the south, but the shortest sea course to reach it is to sail straight north from Minoc.
Sister Antos can be met in the secret room west of the throne room. She directs the Stranger to Garam for details about the bell. In case you need specific directions, you can also ask the guards Lori and Samuel. Garam reveals the exact coordinates where the Bell of Courage is lying; therefore, you need a sextant (see below).
More dialogue threads exist in Serpent's Hold. Allen from Cove said that you will need a magic ship wheel to reach the entrance of th Abyss; more on this below. Thevel from Britain said that a beggar in Serpent's Hold knows the secret of the magic orbs: in fact, Roderick can be met at the castle gate.
Two training rangers start an internal dialogue thread. They direct the Stranger to their trainer Shyra for information about the dungeon altar rooms. She will reveal some information and (in the Master System port only) direct the companions to the treasure guards, who should be asked about "dungeons". Eventually, they direct the Stranger to Durham, who completes the information from Shyra.
In order to reach the exact coordinates where the Bell of Courage is located, you need a sextant. Sailor Sam of Trinsic said to ask to the barkeep in Jhelom. Leave a 99 GP tip, and she reveals how to obtain it from a guild shop. The details depend on which port you are playing, but the explanation is extremely precise.
There are two guild shops: one in Vesper and one in Buccaneer's Den. The former is easier to reach. Once a sextant is purchased (for 900 GP), you can see the party's coordinates with the [L]ocate command. Sail the ship to align exactly with either latitude or longitude, then turn at 90° and go straight to the coordinates revealed by Garam of Serpent's Hold. Search the location and retrieve the Bell of Courage.
The magical ship wheel
Allen of Cove said that you will need a magic ship wheel to reach the entrance of th Abyss. There is a double secret passage in Serpent's Hold: from Sentri's room to either side, then from that secret room to the north. There, Noxum the nixie reveals that His Majesty's Ship 'Cape' had a magic ship wheel, indeed, and relays the stranger to the injured Lassorn for details about the location of the shipwreck. The surviving sailor says that His Majesty's Ship 'Cape' sank in the bay of Cape of Heroes.
If you ask Lord British about Trinsic, he will say that it is north of Cape of Heroes. From Serpent's Hold, sail west following the north coast of the islands. Pass the Cape of Heroes (the southmost tip of the mainland) and search at the centre of the deep waters of the bay.
Use the magical ship wheel whenever the party gets onboard a ship, and it will raise the ship's hit points from 50 to 99. Whenever the ship gets damaged, use the magical wheel again to restore all its 99 HP instantly.