Here Comes the Groom

You can obtain this contract in Svorlag on the Isle of Spikeroog. Speak to Kevan at the harbor to haggle and accept the contract. He'll send you to speak to his sister, Britt, on the other side of the harbor across the bay. After speaking to her you'll have a marker pointing to a cave on the southwest end of the isle. You can follow a trail all the way to the very end to find the mouth of the cave opening up towards the sea. There you will get the objective to explore the cave, which will contain a few large groups of drowners. Using your witcher senses look for the Siren trail and follow it to the center of the fiirst part of the cavern where you will be able to examine a large shrine which gives you the Bestiary entry for Melusine and allows you to contine the investigation. Follow the second part of the trail out to end of the cavern where you will have to climb up and out. Outside, again using your witcher senses, locate the body of the betrothed and examine it for a short cutscene and the discovery of the monster you're here to slay. Coat your blade, cast Quen, and blast with Aard or some Grapeshot.