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 receive this quest from Crach in Kaer Trolde in Ard Skellig at the end of the quest The King Is Dead — Long Live the King.
Walkthrough
For this quest you’ll need to search for Crach’s son Hjalmar, who sailed off to Undvik to kill an ice giant, but who hasn’t been seen since. To learn more about Hjalmar’s expedition, you should visit the New Port inn in Kaer Trolde. The patrons there will tell you that Hjalmar gathered up a bunch of hearty men to go with him, but that he didn’t seem to have much of a plan. They’ll also mention that the ice giant is working with sirens, but that sirens are afraid of something called a 
Hornwall Horn.
When you sail to Undvik in The Skellige Isles Southwest, you should head for the northern part of the island. That’s because when you land on Undvik, your objective will change and ask you to search the ruined town of Urskar, but if you head there from, say, the west rather than the north, you’ll miss some parts of the quest.
So starting from the northern part of Undvik (#1), you should head southeast and then circle around to a large, unfinished ship, following the path shown on the map above. Along the way, if you keep your witcher senses on, then you’ll spot some sea graves, some dead bodies at an avalanche, and evidence that someone dragged a litter along the path.
Eventually you’ll reach the ship (#2), and you’ll witness the ice giant feeding his pet sirens. Once the giant leaves, you should defeat the sirens in the area and then explore the ship. In the hold, you’ll meet a man named Octo (actually Harald Houndsnout, the former jarl of Undvik). He’ll be a bit crazy, but he’ll tell you that he saw Hjalmar heading for the old guard tower to the west. He’ll also mention that he’s been building the ship, and that he could use 
Nails and 
Twine if you find some.
At the guard tower (#3), you’ll find the remains of some of Hjalmar’s men, and you’ll also spot some tracks leading away to the west. When you follow the tracks (examining them every so often to make more appear), they’ll take you to a troll cave, where you’ll find one body out front — with a Hornwall Horn right next to it — and where you’ll spot signs that another man was dragged inside.
Note: If you equip the Hornwall Horn, then you’ll be able to use it to knock sirens out of the sky.
Inside the troll cave (#4), you’ll spot a trail of blood. When you follow it, you’ll eventually come to three trolls by a cooking pot, where they’ll be marinating the surviving man for their next meal. You’ll have two ways to deal with the trolls. You can simply fight them, or you can answer one of their riddles (“troll” and then “rock”). Either way, you’ll rescue the man Folan, who will begin following you. Saving Folan will earn you 200 xp.
When you reach Urskar (#5), you’ll spot signs of a battle, and you’ll detect that Hjalmer’s men fled while being pursued by a giant. When you follow their tracks, they’ll lead you to a cave to the south. Inside the cave (Exit A), you’ll find more evidence of fighting, but you won’t run into anything living other than some harpies, erynias and devourers. So you’ll just need to make your way through the cave until you reach its end.
When you exit the cave, you’ll arrive at the Clan Tordarroch Forge (Exit C). As you look around the site, you’ll discover the ice giant’s “pantry,” complete with a few of Hjalmar’s men hanging around for future use. You’ll then spot some movement. When you head in that direction, you’ll encounter three devourers. Afterwards, if you have your witcher senses on, then you’ll detect some human-sized and giant-sized footprints leading east. As you follow the footprints, you’ll eventually come to a fork in the road with the human-sized footprints going left and the giant-sized footprints going right. You should follow the human-sized footprints.
The human footprints will take you to the Dorve Ruins (#6), where you’ll find Hjalmar fighting a pair of sirens. After helping him with the battle, Hjalmar will tell you that he doesn’t care what’s going on in Kaer Trolde. He’ll insist that he needs to kill the ice giant so he can prove that he’s worthy of being king. You’ll decide to help him.
Note: Everything in the quest leading up to the Dorve Ruins is optional.
Next, Hjalmar will lead you to the ice giant’s cave, but he won’t get too far ahead of you, so you can take the time to explore the Dorve Ruins and do some looting first if you want. Scattered around the ruins you’ll find special Nails in piles of planks. If you take some of these Nails to Octo (#2), then you’ll earn 200 xp. Twine isn’t actually needed.
Inside the ice giant’s cave (#7), you’ll find Vigi, one of Hjalmar’s companions, locked in a cage. Hjalmar will note that Vigi is called “the loon” for a reason, and he’ll recommend that you leave him locked up for the fight, but the choice is yours:
- If you want to free Vigi, then you’ll need to find the key to his cage without stepping on any snow (which will wake up the giant). You’ll find the key in a chest on the southern side of the cave. If you turn on your witcher senses, then the chest will glow red, making it easy to spot. However, when you unlock Vigi, he’ll charge right at the giant and kick him in the head, and the fight will be on. Freeing Vigi will earn you 200 xp.
- If you leave Vigi in his cage (either on purpose or because the fight starts up before you can free him), then he’ll get killed during the fight.
When the battle against the ice giant starts up, you should be careful. The ice giant is big and tough, and he hits hard, but he’s also slow as molasses, so you should be able to dodge around him. Also be sure to use 
Ogroid Oil on your silver sword to enhance your attacks, and cast Quen just in case the giant hits you. Once you’ve hit the giant for about a third of his health, he’ll grab a ship’s anchor and start swinging it at you. Luckily, this will make him even slower than he was before, so you should have an even easier time dodging him.
When you eventually deal enough damage to the ice giant, a cut scene will start up where Hjalmar will land the killing blow. This will net you 100 xp. Killing the ice giant will also complete the quest and earn you a Mastercrafted Silver Sword and 500 xp. Hjalmar will then ask you if you’re ready to leave. If you say no, then he’ll move over to the Marlin Coast (#8) and wait for you. When you talk to Hjalmar, he’ll agree to return to Kaer Trolde.
Note: If Vigi and Folan survive, then they might help you later in the game at Kaer Morhen.
