The Sunstone is a main quest in Skellige and part of Battle Preparations. Eredin, the King of the Wild Hunt, travels on the Naglfar, the spectral ship from Skellige legend, somewhere between worlds. To defeat him, the heroes have to summon him back and make sure he cannot escape again, and that takes a group of mages working together with the help of a legendary elven artifact, the Sunstone.
Finding it is up to Geralt. The trail runs through Skellige and its people, and where the stone ends up depends on how the succession to the throne played out. Depending on who rules the isles, the search can take Geralt to Kaer Muire, the village of Arinbjorn or a cave under Eldberg, and it involves old friends such as Mousesack, the an Craite family and the new ruler of Skellige.
It is a quest that rewards the time players have spent in Skellige: the choices made in the fight over the throne come back here and decide how Geralt gets his hands on the Sunstone.