A Bard's Beloved
Side quest
Skellige suggested level 15 
On the world map
From the journal
Geralt happened across a terrified bard who trembled when he spoke of his "beloved", yet other than that refused to say an ill word about the woman. Even stranger, everything seemed to indicate his betrothed lived in a cave. Geralt decided to check out what sort of curious couple he was dealing with.
It turned out the bard's "beloved" was a hideous water hag. No wonder the thought of her caress made him shake in fear and revulsion. Geralt dealt with her as he usually deals with monsters.
While traversing the charming yet dangerous Skellige isles, Geralt happened across a mysterious cave. His instincts told him there would be more than bats and newts dwelling within - and so he decided to enter it.
It turned out the cave had become home to a water hag who would lure victims in order to kill and eat them. Geralt dealt with this evil monster as it deserved and continued on his way.