This interesting quest takes place in Tartarus and is a test for even the best.
Talk to the White Witch who lives on the island of Westernesse. To find her, go to the main docks of Westernesse and use the boat there by the sign to 'Cross' (similar to the Skara Brae crossing) to her floating Coven.
The Witch will test you before offering you a chance at her ultimate task. If you succeed in everything she asks you will obtain a valuable Token. When you use the Token you can select the Artifact of your choice.