Possession - Skellige
Location: Kaer Trolde Keep
Quest Giver: Crach an Craite
Other Requirements: Complete this quest before The Isle of Mists or it will fail
This quest begins after speaking with Crach an Craite during The King is Dead - Long Live the King main story quest. You must complete this quest before doing The Isle of Mists story quest; completing The Isle of Mists will automatically fail this quest.
For this quest the first thing you will need to do is travel to Svorlag and speak with the Udalryk who is currently having an argument. If you choose the wrong dialogue option you won't be able to ask him about Cerys, instead someone else will interrupt your conversation. If this happens to you, you'll have to ask the villagers about Cerys (as pictured above).
The villagers you have to ask about her are all around the Signpost in the central portion of town. Once you've spoken to the regular towns folk (all marked on your compass) speak to Bergthora to update your quest again. She will point you to a place due north of where we currently are.
When you arrive Eiric will be atop a rock yelling for help surrounded by Drowners. As always, choosing the Axii option will net you experience for the dialogue, but choose whichever one you want. Our next stop is a small abandoned town to the southwest of our current location.
Inside the large building here you'll find Cerys as well as.... lots of weird shit going on. After speaking with Cerys your quest will update again and she'll ask you to find the Brokvar sword. The sword is found in the cellar but in order to reach it you'll need to inspect the backside of the oven first for the cellar key then go downstairs.
You'll find Brokvar by inspecting one of the tables down in the basement, it is one of the only locations that glows using your Witcher Senses so it shouldn't be too hard to find. When you pick up the sword you'll see a shadow cast on the wall of a Hym, which is the type of monster causing all of this trouble.
Return to Cerys and speak with her then follow her to Udalryk to fill him in on all you have learned. When the quest updates you'll be sent out to a place underwater southeast of the town. Right next to this location there will be a Hidden Treasure (body on the small island by a rock, loot it for a key then the locked chest is in the water - right near our quest objective). This quest is called Not Only Eagles Dare and I recommend you grab the treasure/do this small quest while out here.
Place the sword next to the body underwater when you're ready to proceed with the quest and then return to town. There will be a short cutscene followed by Geralt explaining to Cerys that a Hym is the problem and there are two ways to deal with it.
The Witcher way involves pulling the Hym out and fighting it. Cerys way involves tricking it and is the faster of the two methods since it is mostly all just dialogue from here until the end of the quest. At the end of your dialogue with her she'll give you the option of fast traveling to the haunted house or meeting her there later. Choose whichever, just go to the house when you wish to advance the quest forward.
At the house you'll need to search the two rooms in the quest circle after which Cerys will call you and tell you she has an idea, but you have to trust her. If you fail to trust her and give the baby to the Udalryk then you'll automatically resort to the other method of completing the quest, which involves drawing out the Hym and fighting it.
If you choose this method you'll need to collect four torches which you need to place in the house to draw the Hym out. During the fight with it you'll need to use Axii on Udalryk to calm him down from time to time; when you get the Hym really low in HP he'll flee into the cellar, take care of him down here to complete the quest.
Hym Weaknesses: Moon Dust, Specter Oil and Igni