Witcher 3 X Marks the Spot Walkthrough
Start Location: This can be started inside a small shack just to the east of the lake south of the ‘Blandare’ fast travel marker in central Ard Skellig. This is also very close to the southwest of the Hidden Treasure marker associated with the ‘Freya Be Praised’ side-quest.
After locating the shack to the east of the lake, head on inside. Once you are in doors, use Geralt’s Witcher Sense to examine the human remains in the middle of the floor.
Upon looting the body you’ll receive a Mysterious Map’ and an Old Key. Read the Mysterious Map item in the Key Items section of the inventory. This will trigger the ‘X Marks the Spot side-quest. Note that looting the objects will have a Wraith spawn outside of the shack, so be prepared for a fight as you leave.
Make your way to the objective marker to the west, just along the road to the southeast of the town of Fayrlund. Inside the search area, locate the small stream with the waterfall and make your way down to it. Behind the waterfall is a small cave containing a chest.
Use the Old Key on the chest here to unlock it and loot it to complete the side-quest. It contains a Runestone, and some other goodies.