When did you report to Marcus? He has to get the cave letter first and the sewers letter afterwards. The cave letter triggers the "Speak to Tingalf option".-> speak to Tingalf, get clue for new lair in sewers -> enter sewers -> get sewers letter ->report to Robin ->report to Marcus.
Kiya <hating this quest <img src="/ubbthreads/images/graemlins/puppyeyes.gif" alt="" />>
If both, Robin and Marcus, only have standard sentences and Trevor/Verdistis has nothing new/interesting to say = quest sequence is busted.
