I'm having this same issue. Ran into Tarquin outside the graveyard a few levels ago and accepted his quest. Then I realized I screwed up something else, reloaded, and played through a different order of quests without talking to Tarquin. I showed up at the graveyard expecting to find him to get the quest, and he's not there. He IS on the Lady Vengeance near the boat that takes you to shore, but only has trade dialogue. This quest needs a lot of help. :(
