Information about finding the Hare Mask in The Witcher 3 Blood and Wine.
During the quest The Beast of Toussaint you’ll track down the Beast’s fourth victim, who is targetted in part because of the cowardly behavior associated with the animal he’s decorated as for a silly event known as the “Hare Hunt”. Toussaintois are weird folk. Afterwards you’ll play through the entire quest Blood Run, which to be fair consists of only a cutscene, a boss fight and a conversation, and you can’t even get a game over from the boss fight.
Once that’s done you’ll be free to explore Toussaint, but before you go seeking out any quests, there’s a bit of loot you can get. The Beast’s fourth victim was dressed in - among other things - a Hare Mask, and if you’re quick and return to the scene of the crime you can claim this prize. If you run off to do something else, the loot will vanish, so either you do it now, or you forget about it.
After Blood Run you’ll get control of Geralt in the Warehouse where you confronted the Beast, on the western coast of Beauclair. Cross the river to the west to return to the palace gardens, where you’ll need to look for a greenhouse near the river. You can actually spot these structures on the map - you’re looking for a rectangular building near the river just east of the Palace Gardens signpost, which itself is on the southern end of the gardens near a stream.
Enter the greenhouse and search the ground for a generic loot pile - the body of the victim hasn’t been left to suffer the indignity of exposure, it seems. The bloodstains on the ground make it pretty easy to spot you’re in the right place, and the Hare Mask should be nearby.
What does the Hare Mask do? Why is it so important? Nothing, and it’s not. It’s just a decorative mask you can wear, albeit one that’s highly missable. If you like playing dress up, it’s worth the detour, if not… well… nobody’s perfect.