This is the famous CSI: Skellige case. Use Witcher Senses to look at anything you like, including the corpses of the fallen, but what you want to check out is one of the bears. Look at everything to determine that the bear is a shapeshifter, and particularly check out its mouth.
Your next goal is to check out the empty mead horns, one of the mead bowls present at each table, and the mead bowl closest to the door. Geralt concludes that the mead was drugged.
Speak with Cerys, who suggests checking in with Hjolt; follow her to the seer. He identifies the substance used, and Cerys suggests you explore the cellar where the drinks are stored to see if you can figure out where the drug was introduced to the mead supply.
Follow Cerys into the cellar, and use Witcher Senses to find the correct barrels. As soon as you find them, someone sets the cellar blaze; with all that strong spirit around, you’re likely to be killed. Look where Cerys is standing to spot a stack of barrels in front of a tunnel; use Aard to clear them, then get inside quick. As you pass through the tunnel, don’t forget to draw on the Place of Power you pass on the way.
Back in the hall, you return to the cellar entrance to try and track the arsonist. Luckily, they broke a bottle and tramped through the puddle; follow the footprints till they dry out, then look around to find a piece of cloth torn off and hanging on a wall sconce. Investigate this and show it to Cerys to pinpoint the culprit – Arnvald.
When you go after Arnvald, he flees – jump on Roach and chase him down. You need to cut him down with a sword to proceed; don’t worry, he survives. When you aqtch him, interrogate him and he reveals the masterind behind the plot – Brina Bran.
If Aarnvald evades you all the way to the beach, you’ll have to battle a bunch of his mates before you can speak with him. If you’re really late they’ll have killed him, and you must search his body, and then his room back at Kaer Trolde, for evidence of his betrayal. Look for an envelope with a scent trail to find a hidden compartment with the letter you need.
When the investigation is complete, you meet with the Jarls to discuss what you’ve discovered. The evidence isn’t enough to convict Brina Bran, but her son, Svanrige, is an honest sort and speaks out, horrified at his mother’s actions. The quest is complete, and you can continue to Coronation.
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