The Cave of Dreams - Skellige
Location: South of Kaer Muire
Quest Giver: Blueboy Lugos or Madman Lugos
Other Requirements: Complete before King's Gambit
You can accept this quest one of two ways, being sent here by Madman Lugos or stumbling upon Blueboy Lugos and his men in your adventures. If you've completed the King's Gambit quest then this quest will be unavailable to you. My guide assumes that you were sent here by Madman Lugos in the Stranger in a Strange Land quest.
When you arrive at Blueboy Lugos' you'll have to either force your way onto the ship or use Axii on Jorulf, one of the guards here. You can tell Blueboy Lugos that his father sent you or not, it doesn't matter. After some dialogue you'll find yourself in front of the cave when you regain control of Geralt.
Just inside the cave you'll fight some Nekkars after which you'll have to eat some herbs which will make you hallucinate. You'll be teleported to a new cave where you will have to witness each person in your parties worst fears. Follow the white dots on your compass and each event is pretty straight forward.
Geralt's trial is against Eredin, the king of the Wild Hunt and once you defeat him the trial ends and you're awaken by Blueboy Lugos and his friends. He'll tell you that your debt is paid off (if you were sent here by Madman Lugos) or you can demand payment from him, if you stumbled upon him randomly.