Castro

The Treasure Hunters are four friends from Britain, who had gone into the dungeon Despise in order to find the legendary Kiran Shield, with the Kiran Stones as the key. However, their quest quick turned to disaster.

Castro and Felix
On their search for the yellow Kiran Stone, Castro and Felix got caught by the brainwashed Iolo, who put them into a prison cell near the entrance of the dungeon. They were imprisoned in there for almost a week, before the Avatar found them there in Ultima IX, hearing through the door their argument and complaints how hungry they were.

Once freed, they told the Avatar the sotry of Kiran and that they'd come into the dungeon after they had been thrown out of Britain thanks to mayor Aidon's policies. They revealed the position of the yellow stone and that their two friends had ventured deeper into the dungeon, before leaving to get something to eat.

Mourdin
Mourdin had actually managed to find the blue Kiran Stone and even hid it quite well. But then he had a really bad idea and drank from a poisoned fountain and died on the spot from it. The Avatar found his body and, following his instructions, also the blue stone.

Thadious
Thadious had managed to venture the deepest into the dungeon. But right when the Avatar had managed to catch up with him, he was threatened to get crushed in a trap. Barely saved by the Avatar, Thadious gave up the green Kiran Stone and explained that the stones were a key to great treasure.

He himself however had lost all interest after nearly getting crushed. He only wanted to return to Britain and forget the whole thing.

Trivia

 * It is possible to let Thadious die in the trap. Then however the Avatar has to go into it and get endangered, in order to get the green Kiran Stone.
 * The dialogue about eating a Giant Rat that the Avatar hears through the door is hilarious.