Sartan's Suspicion - Act 3 Companion Side Quest
Location: Kalrathia & Eastern Plain of Tears
Closest Portal: Eastern Plain of Tears
Objective: Speak with Osric in Eastern Plain of Tears then free the prisoners from the Morden Arsenal
Reward: Random Loot & Gold
Sartan's Suspicion is a Side Quest in Act 3 that begins by speaking with Feltan the Drunkard with Sartan in your party. This quest is Sartan's Companion Quest in Dungeon Siege 2, each recruitable character you meet in the game will get their own quest. You'll need to have Sartan in your party (and be controlling him) for each of the task updates throughout this quest.
For the first part of this quest you'll want to travel to the Eastern Plain of Tears Teleporter; from this location you'll want to travel directly east into A Mysterious Cavern. You should be able to see the silver star on the map from the Teleporter location.
With Sartan in your group head over to the silver star on your map for a scene with him and Osric, the same person who locked Sartan up at Windstone Fortress. Use my map above if you need directions, you can see the silver star at Osric - whom is also marked - for the first part of the quest.
Sartan is a bit of a dummy and he one-shots Osric immediately when he sees him. Soldier Orayne will come out after Osric is dead and inform you of your mistake, after which you'll need to go to the Morden Arsenal nearby (marked on my map above too) and free the prisoners there to complete this quest. In order to free the prisoners destroy the bombs right next to their prison cell and it'll blast open.
Your reward for completing this quest is a Big Chest which you'll find in one of the prison cells. Sadly, you won't get anything when you return to Osric after freeing the soldiers for the final update of the quest.