Apparently I'm not alone on this. Since giving him both pieces, I've been consistently going back to Tarq to attempt to forward the quest but he constantly says he just needs a few more items. Skull powder, illweed, etc., all of which are not items you can acquire... so he just wanders around in front of the graveyard gate. The Quest "All in the Family" is completed with the last entry "We gave the pieces of Anathema to Tarquin" and that's it.
Lovely bug... holding out hope someone will find a solution or there will be a patch.
