I have a similar problem in that Tarquin was never outside Stonegarden in the first place. He just stayed on my ship. I then went exploring and found the pieces to the weapon, but didn't even know it was a quest until I read about it on forums. I have no journal entries for it at all and can't do anything with Tarquin aside from trade with him. It really sucks because from what I've heard it plays a role in how the other acts play out in some major way.
