Disgusting wight key is a quest item and a key in the Blood and Wine expansion.
It can only be obtained if Geralt kills the Spotted Wight of Trastamara Estate Ruins, rather than lifting the curse, during the main quest La Cage au Fou. Despite it being a quest item, it's not actually needed to do anything noteworthy during the quest. Instead, it opens the doors for a tunnel with some minor loot between the cellar and a secret exit to the north of the house.