The Treasure Hunt: Tough Luck is a Side Quest in Velen.

Travel south of the Devil's Pit and down the path with hanging corpses, next to the unfortunate events treasure.

If you haven't gone down this path before you will run into mercenaries attacking a caravan. The five men are level 5's and there is one enemy with a shield, use your AXII to get behind him and kill him. Once you've defeated the men the traders will give you some coin and you will gain experience.

Now back on the treasure hunt, travel past the traders cart and go into the forest. Eventually, you will see a cart with a foglet guarding it. Before you can claim your treasure you have to defeat the foglet.

When you approach the foglet it will dissappear into a fog. The fog will move around and when the fog approaches you, dodge-sidestep out of the way, as the foglet will come out of the fog swinging. Hit the foglet and remember to use Quen and Igni. After hitting the foglet it will go into another fog, then rinse and repeat until it is dead. Take note that the foglet can summon two beings that will go into fogs too, so give them one quick hit to make them disappear.

Once the foglet is defeated loot the body on the wheel of the cart and read the letter. After reading the letter, loot the women's corpse near the pond to get the key to unlock the chest in the back of the cart. Once you unlock the chest, loot it and you will complete the treasure hunt.