Head to Ulula's Needle on Spikeroog to find a hidden treasure marker and several mucknixers. Once they're killed off, loot the body here for a note and key and read the note to update the search area. Note that in some cases, a drowned dead will spawn nearby once you killed off the mucknixers but if you're far enough away, it won't automatically target you so sometimes you can avoid it here.
Thankfully the chest is nearby on the outer edge of the search area, but depending, you may run into a siren or two here. Once you loot the chest the quest will complete.
- 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.
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.
- Read the note you found.
- Find the underwater treasure using your Witcher Senses.