This guide is a walkthrough of a side quest in the Witcher 3 on the Skellige Isles. The guide is composed of a map, map legend, and walkthrough to show readers exactly where to go and what to do to complete the quest.
Background
You’ll automatically trigger the quest “King’s Gambit” once you’ve helped Crach’s children Cerys and Hjalmar in the quests Possession and The Lord of Undvik.
Walkthrough
After triggering the quest, when you return to Kaer Trolde (Exit A), you’ll meet the steward Arnvald at the entrance. He’ll lead you to the feast hall (#1), where you’ll witness Cerys and Hjalmar arguing with each other. During the ensuing conversation, Hjalmar will mention that there are some vildkaarls in attendance.
Before leaving the feast hall to find Crach, you might want to visit Hjalmar in the northwestern corner (#2). He’ll be boxing — and losing — against some of the vildkaarls. When he sees you, he’ll invite you to take your chances with them. If you win your bout, then you’ll receive a leveled version of Hjalmar’s Steel Sword plus 100 xp. You’ll also get a chance to fight three vildkaarls in a row. If you beat them, then you’ll earn an additional 50 xp.
When you eventually approach Crach’s chamber (#3), you’ll be just in time to see Birna Bran leaving. She’ll inform you that voting for monarchs is silly, and that the succession should stay in one family — hers — with her son Svanrige becoming the next king. Then she’ll depart. Then when you enter the chamber and talk to Crach, he’ll give you your reward for helping his children: a leveled version of the silver sword Fate plus 500 xp.
At the end of your conversation with Crach, you’ll hear a commotion in the feast hall, and when you get back there you’ll find the place under attack by three level 18 bears. Defeating the bears will earn you 300 xp. At the end of the battle, speculation will run rampant that somebody was trying to kill the claimants to the throne, and Crach’s children will have differing ideas about how to deal with the matter. Hjalmar will want to take direct action against the culprits while Cerys will want to start an investigation and figure out what happened. You’ll then have to choose one of the two. The quest will change depending on the one you pick, and your choice will eventually become king / queen.
Note: If you pick Cerys, then you’ll learn who orchestrated the attack, and you’ll gain access to a place of power beneath Kaer Trolde. If you pick Hjalmar, then you won’t and you won’t.
Cerys
The first thing you’ll need to do in Cerys’ part of the quest is search the feast hall using your witcher senses. The multitude of dead bodies won’t reveal anything to you, but when you examine the bears, you’ll discover that they’re part human, and you’ll decide that the “earthy” mead they drank must have caused the transformation. You’ll then need to examine all of the drinking containers. From one you’ll detect the scent of a mushroom, and when you follow its aroma you’ll find a drinking horn containing a mixture of mead, blood, and the mushroom.
When you show the horn to Cerys, she’ll suggest that Hjort might recognize the mushroom. To reach Hjort, you’ll need to exit the feast hall via the eastern exit (Exit B) and find him in the lower courtyard. Hjort will inform you that the mushroom is called mardroeme (aka psilocybe) and that it can transform warriors into beasts when they’re consumed by battle rage.
At this point, Cerys will insist that there’s no way the mead could have been tainted in the feast hall — that someone surely would have noticed — and so she’ll suggest that you check out the store room in the cellar where alcohol is kept. She’ll then lead you to the cellar (Exit C).
Inside the store room (#4), you should turn on your witcher senses to make the mead barrels glow red. You’ll then need to examine the barrels until you find the tainted one. However, when that happens someone will sneak into the room behind you and light all of the spilled alcohol on fire. You won’t be able to put the fire out or escape the way you came. Instead, you’ll need to use your weapon or Aard to knock down some barrels and flee the room via an escape tunnel (Exit D).
Cerys will lead you through the escape tunnel, which will eventually circle around and lead you back to the store room entrance. During the trip, you’ll pass by a Place of Power (#5). You’ll only be able to access this Place of Power during Cerys’ path through the quest. If you choose Hjalmar then you’ll miss it.
When you return to the store room, you’ll need to turn on your witcher senses and examine the entrance. You’ll find a broken bottle of mead on the floor, and you’ll notice that the culprit stepped on it, leaving tracks behind. When you follow the tracks, they’ll lead you to a wall sconce, where you’ll discover a piece of the culprit’s shirt. Cerys will recognize the fabric as belonging to Arnvald. This discovery will earn you 300 xp.
You and Cerys will then race back towards the entrance hall, where you’ll spot Arnvald trying to escape on a horse. Conveniently, you’ll suddenly find yourself mounted on Roach, so you’ll just need to chase Arnvald and then hit him with your sword to knock him down. During the ensuing interrogation, Arnvald will give up Birna as the mastermind of the plot, and he’ll give you the (unsigned) letter she sent him with instructions for how to poison the mead.
Note: If you’re unable to knock Arnvald off his horse, then you’ll have to follow him or his horse’s tracks to a beach near the Crossroads. Arnvald will meet his “allies” there, but they’ll try to kill him. If you save Arnvald, then the quest will proceed in about the same way as if you had knocked him off his horse. But if Arnvald dies, then you’ll have to loot Arnvald’s Key from his corpse and search his room in Kaer Trolde (#6). You’ll find a scented envelope there, which will lead you to Birna’s incriminating (but still unsigned) letter.
With your investigation complete, you’ll earn 200 xp, and the jarls will convene at Kaer Trolde. In a roundabout way, Birna will be found guilty, and she’ll be condemned to death.
Hjalmar
Hjalmar will take you to the vildkaarl village of Fornhala, and he’ll inform you that the vidkaarls can transform themselves into bears. However, when you explore the village, you won’t find anybody there — living at least. But eventually you’ll spot an altar, and Hjalmar will determine that it belongs to the god Svalbold, who was banned years ago because he requires human sacrifices from his followers.
While you’re at the altar, you’ll detect the smell of incense. The odor will lead you to a pool of blood mixed with bile, and you’ll see that somebody walked through it. When you follow the footprints, they’ll take you to the house on the southern edge of the village, where you’ll find a trap door leading down into a cave.
Inside the cave (Exit E), you’ll meet a level 17 vildkaarl named Halgrim (#7). He’ll tell you that “we didn’t come to the feast to kill,” but he won’t sound sorry about what happened, either. Then Hjalmar will attack him, and he won’t say anything more. Halgrim will start out in human form but then transform to a bear once you’ve done enough damage to him. When you loot Halgrim’s corpse, you’ll find some bear parts plus a 
Berserker Hide. You’ll also earn 200 xp during the encounter.
Past Halgrim, you’ll witness the vildkaarls performing a ritual (#8). When you reach the ritual chamber, you’ll meet the druid Artis. He’ll tell you that he’d rather die than reveal anything to you, and Hjalmar will oblige him, so you won’t learn anything new. Artis will then attack you and summon bears to help him, but since Artis is a druid wearing robes, he won’t be able to take too many hits before going down.
When Artis dies, you’ll find a letter on his corpse indicating that he was working with someone and that he was going to become the new king’s adviser, but the letter won’t give any hints about who sent it, so you won’t learn who was responsible for the massacre. However, you’ll still earn 300 xp for dealing with the vildkaarls.
You and Hjalmar will then return to Kaer Trolde. The jarls will be content with Hjalmar’s actions, and the an Craites will be cleared, but the guilty party will get away.
Afterwards
After completing your investigation with Cerys or Hjalmar and clearing the an Craite’s family name, you’ll meet with Crach at Kaer Trolde. Your conversation with him will complete the quest “King’s Gambit” and reward you with the crossbow 
Death from Above and 400 xp.
The quest “Coronation” will then trigger. To complete it, you’ll just need to travel to Gedyneith to see Cerys or Hjalmar (whichever one you helped during the quest) crowned as the new ruler of Skellige. Watching the short ceremony will earn you 200 xp.
