Echoes of the Past - Skellige
This quest is automatically given to you immediately upon completing The King is Dead - Long Live The King Story Quest in Skellige. First thing you'll have to do for this quest is meet up with Yennefer who is far to the southeast from where you first arrived on Skellige.
You'll have to go through some dialogue here after which you will have to run a small distance to meet back up with Yennefer. Help her battle the foglets (pictured below) then you'll have some more dialogue, after which the Mask of Uroboros will be in your quick list and able to be used as a quest item.
For the next part of the quest you will be following Yennefer through the forest and using the Mask of Uroboros in the areas she tells you to. Throughout your journey you'll be battling a few Foglets but that's about it. After you examine all of the areas that require the mask Ermion will come out and yell at you. Tell him to bugger off and head south to check out the last area which Yennefer will help you out with by moving a tree.
Here you'll find a body of the Wild Hunt and the quest will complete. There will be a bunch of dialogue followed by Yennfer asking you if you wish to fast travel to the next quest location immediately or meet her there. Missing Persons is the name of the next quest and it's another Skellige story quest.