Not Only Eagles Dare
Treasure hunt
Skellige suggested level 10 
On the world map
From the journal
The crooked mast marks the spot - but not just any fool can find the treasure.
You can say a great many things about Geralt, but one thing is certain: he's no fool. He found the treasure mentioned in the mysterious note quite easily.
While in the Skellige Isles Geralt came across a riddle hinting at the location of a treasure. While he was not particularly fond of riddles, he was quite fond of treasure, so he decided to investigate.