Is a Skellige side quest with suggested level 18.

You can pick up this quest from the Kaer Trolde notice board. After picking up the queste, talk to the Bjorg for more information on the contract, and increasing your reward amount.

After talking to Bjorg, to talk the other witnesses for more information about the monster. After talking to the two witnesses, head toward the location of the drowner attacks.

Once arriving at the location, use witcher senses, and examine a nearby dead body. Deal with the drowners you come across, and follow the scent from the dead body. The scent will eventually head underwater, so you will need to dive underwater and into an underground cave.

Keep following the path using your witcher senses. Inside, you will eventually come across an area with more drowners that must be dealt with. After dealing with them, investigate the area with witcher senses to determine more information on the monster.

After examining all the clues, Geralt will determine that it is a Water Hag. This water hag has a keen sense of smell and won't come out when it smells Geralt, so you will need to use "Drowner Pheromones", to hide Geralt's scent, and draw out the monster.

After using the pheromones, hide and the water hag named Abaya will appear, along with some drowners. After dealing with Abaya, loot the corpse, and return to Bjorg to collect your reward. Bjorg won't be happy to pay for the reward.