Passengers: Pierre and Shaine
You have to play the role playing game with them. I'm not sure which dialogues are necessary but I tried to act as the in game character as much as possible. You are a thief so I chose to observe the surroundings, pick a lock etc. Then you should choose to fight Xenofex and the last important choice is to use a ring. After that you should beat Xenofex and the achievement unlocks just before the passengers leave.