This quest is also related to: Down the Hatch.
1: Travel to the Slayer camp near the Champion Harbour
2: Acquire a key from Tilian. This can be done in several ways
-You can tell Tilian you're going to free Orobas, then kill him (and the other Slayers) and loot the key.
-You can simply trade Lord Arben's sword for it.
-Lastly, you can read Tilian's mind and use the information to convince him to give you the key.
3: The Chest is located up the top ramp above the room with the portal.
Reward: 4500xp, 480g + 1 (1125xp, 480g, 3 magical items, 1 other)