Is a Skellige side quest with suggested level 19.

You can pick this quest up from the Svorlag noticeboard. After picking up the quest, head to Kevan for more information on the contract, as well as increasing the reward amount.

After talking to Kevan, go talk to Britt. She will give more information on the monster. After talking to Britt, start heading towards the cave area she mentioned.

As soon as Geralt arrives, he immediately determines that there are necrophages involved, and there will be lots of drowners inside this cave. But they couldn't have lifted Nils (the groom) into the air. Further investigation, Geralt thinks it might have been a Siren who took Nils.

Keep using your witcher senses to follow the trail. When you come upon a shrine, Geralt determines it is actually an Ekhidna named Melusine. In the next room there will be more drowners to deal with. After dealing with these drowners, there is a cave opening you must climb up and out of, before doing so it would be a good idea to use your "hybrid oil" for the upcoming fight.

Once outside, you'll see the dead body of Nils, and then the fight will begin.

After dealing with the monster, loot it, then return to Kevan to collect your reward.