I totally broke this quest. Haven't even been to Verditis? and have already exposed Tingalf to Marcus via the sewer letter only. Got the skill upgrade though. Later I found the cave out by the water supply and that letter to Marcus. He didn't have anything to say but I got experience for it anyways.
My problem is I hate black on my map so I try to totally clear one map (Riverton) before going on to the next.
