Hi.
even the way the quest is bugged in my game differs, I picked up this thread to describe it, because as far as I could see it's the latest in this forum referring to this topic.
When leaving Reaper's Coast Tarquin was not on board as he was supposed to be. Before it seems that the quest went fine. I brought him both pieces of Anathema at the gate of Stonegarden and the in the journal the quest was closed properly.
But he never left Stone garden. When talkig to him he only claims he needs this and that, collecting ingredients etc. and he never showed up on the ship. After proceedeing to and arriving at Nameless Isle he still was absent. I just continued to and who's missing - Tarquin.
Now I'm at Arx and will continue til final of the game. Not sure if he'll be there to pray with Maladie and Almira so that using source skills will be free of cost (source points). If not, the final battle will be quite a challenge.
I thing the weapon itself is not decisive. The first walkthrough I finished without it
