This Tartarus quest actually begins in the Vintner's shop beside Yhaguen's Trammel house. You can get there by using the Moongate near the south east corner of the Arete Vendor Market.
Thieves have been stealing Yhaguen's grapes. This has greatly upset Tully (his vintner). Talk to Tully to begin this quest and return a thing of rare value to him for your reward. Tully has offers special wines and other sought after items. If you are lucky he may have some really nice clothes for you (other goodies can also be obtained along the way).
Tully wants you to punish the grape theives and in doing so you will find yourself on a challenging journey.
The quest can be soloed or you can bring friends to help you. There is a 4 hour cool down period.