The Lord of Undvik - Skellige
Quest Giver: Automatically received during The King is Dead - Long Live the King
Other Requirements: N/A
You receive this quest during the Skellige Story Quest, The King Is Dead - Long Live The King. Typically during your normal progression through this story quest you'll complete the first few objectives of this quest naturally. If you didn't then go to the New Port Inn and ask the Innkeeper about Undvik. This will get you all of the updates you need to continue with the quest.
The next part of this quest will have you go to an island in the southwestern portion of Skellige, an abandoned town by the name of Urskar. Here you will have to use your Witcher Senses to investigate the area, follow the foot prints to the south up the mountain path until you come to a cave entrance. Shortly inside of that cave you'll find an area where you can dive down into some water, do so.
Note: There is another set of foot prints that you can follow out of town to meet up with a man by the name of Folan. He will accompany you on the rest of the quest and you'll have to keep him alive if you choose to bring him with you. Meeting Folan is completely optional, however I feel that it's worth mentioning that if you meet Folan and bring him with you he will accompany you on a major future story quest. For more information about this check out my Brothers In Arms: Skellige Quest page.
Throughout the cave you dove into you'll find some harpies flying around - shoot them down with your Crossbow or swipe at them with your Silver Sword when they get close. I strongly recommend you use a Cat's Eye Potion of some kind to improve your night vision in here. There will be rocks for you to climb up at the northern end of the cave, look for the white markings that let you know you can climb there.
While going up the rocks and bridges you'll come across bodies of Harpies that glow red with your Witcher Senses, examine them if you wish for quest lore. Eventually you will come across a room with a giant tree with Harpies flying around it; you'll find some ladders and scaffolding in here to climb up. They will lead you to some more tunnels which will lead you out to the top.
I recommend you take your time in these tunnels and search them thoroughly, there are a bunch of crafting recipes and goodies for you to find in chests, barrels and crates scattered about. When you reach the top of the mountain you'll get the Clan Tordarroch Forge Signpost waypoint. The Master Armorers Side Quest is also done in this same exact area, if you have that quest now and haven't done it you may want to grab the item while you're here.
Note: If you did do that quest earlier you can fast travel to the peak of this mountain and skip to this part of the quest, cutting out the entire first bit.
On the mountain's peak you will find a building to enter, inside there will be a brief scene of Hjalmir fleeing the area and your quest will update. Follow the foot prints out of the building and down the trail to Dorve Ruins - you'll run into the Signpost along the road, just follow the road and ignore the prints when they leave the trail.
When you arrive at the Dorve Ruins you'll find Hjalmar fighting Harpies in the area. Help him out by fighting off the two Harpies then speak to him to continue the quest. Now we've but one thing left to do - defeat the giant! Follow Hjalmar up the path into the mountain until you reach a small little buttcrack in the wall. There will be a brief scene and within a matter of seconds the whole area will be cleared of rocks!
Inside of the troll's den you'll find one of Hjalmar's comrades in a cage. You'll have a choice to make here, either free him from the cage before fighting the troll or after. If you wait until after you'll be killing him since the cage is destroyed during the fight.
Choosing to free Vigi will require you to sneak around the room avoiding the snow patches - stepping on them will wake the giant. Once Vigi is free, feel free to engage the Ice Giant freely. Note: If you save Vigi here, he will be able to join you on a major future story quest. For more information about this check out my Brothers In Arms: Skellige Guide.
The first part of the fight the Ice Giant will use his fists to attack you, after hitting him a few times he will rush over to the area where Vigi was being held captive and take out a giant chain to beat you over the head with. Failing to dodge is very punishing for this part of the fight.
Ice Giant Weaknesses: Ogroid Oil, Quen
Once the giant is defeated you should turn down Hjalmar's offer to leave - it will put you back in town and not let you loot anything. Return to town once you're done pillaging the cave to turn in the quest for good. That's the end of it. Completion of this quest will put the quest King's Gambit in your log.
Completing of the King's Gambit Quest also will cut you off from doing Stranger in a Strange Land and The Cave of Dreams Secondary Quests. Both of these quests become unavailable once you complete or fail the King's Gambit Quest.