Walkthrough

The Lord of Undvik is a Skellige side quest with suggested level 17, which is obtained after talking to Crache; it involves finding and helping his son, Hjalmar. Note that it's quite long, so don't expect it to be a quick search-and-rescue mission.

To get started talk to Crache to get the quest and get more information on where Hjalmar can be found.

To accomplish the first (optional) objective, go to the New Port Inn and talk the people from Undvik, to find more about the giant, and the Jarl of Undvik. One will talk about how the Jarl died; the other talks about the many Sirens there.

After talking to the survivors; sail to Undvik, or if you have the destination unlocked on your map, fast travel directly there. Once on Undvik, head to the ruins of Urskar as shown on your map marker.

Fight the Sirens

Once you get close; a cutscene will occur, with Geralt observing some Sirens around a half built boat, and the giant walking past, and giving the Sirens some food, as well as putting some food in the boat. Kill any sirens outside the boat, then  climb into the boat, and examine why the giant put food there. Talk to the man inside the boat, who turns out to be the now insane Jarl of Undvik. Talk to him to find out if he has seen Hjalmar; he has, and tells where they are likely camped. Before leaving, he will ask you to bring him Nails and Twine to finish building the boat. You likely have some twine on you -- and there are nails in the some of the boards around the ship, if you want to do him the favor.

Head towards Hjalmar's camp; you may encounter more Sirens along the way. Once at the campsite, use your Witcher Senses to examine the bodies. There are one set of prints leading away from the camp, before the camp was attacked. And one set of tracks with a group of men dragging a boat north.

Rescue Folan From the Trolls

Follow the set of prints leading away from the camp, before the attack, you will discover that one was attacked by an archer. There are set of tracks leading into a nearby cave, which is where the archer was dragged by Trolls. Follow the tracks into the cave. You will encounter some trolls, which you may dodge around or kill. Keep following the tracks and you'll come across another group of trolls, cooking the archer for dinner! You may play "riddles" with the trolls and convince them to let the man go, or show them the "way of steel" and release him yourself.

Once he is released, talk to the man, and you'll find out how he was captured by the Trolls. Named Folan, he will also say the best way to find the giant will be crossing the lake, then into the caves above the village. And another option will be going through the mines. You can team up with Folan and have him come along to finish the quest.

Follow the Tracks

Head back and follow the boat trail that lead from the camp When you arrive at the boat, you will encounter some Wraiths there. Near the boat you will see some dead bodies, and following the trail, some tracks leading away. Follow the tracks and they lead towards the caves, and that the giant chased them but couldn't get inside the cave. Use your Witcher Senses to continue following the tracks, and you will arrive at the cave.

Inside the cave, you will see the trail lead over a big drop. And once you jump into the water below; you'll encounter some Harpy-like enemies named Eyrnias. There are a few loot sacks and chests scattered around the perimeter of the cave, and a corpse with money, and a chest in a water pool if you head straight into the cave. 

From where you jumped into the water; bear left until you find some ledges you can jump onto; take them, and follow a narrow path.  You'll eventually come across several harpies flying around a big tree. You may go towards the tree and fight the harpies, or staying left again; there is a nearby ladder heading up. Climb the ladder, and jump onto the nearby ledge, and go up another ladder. Using Witcher Senses again, keep following the blood trail.

There are at least two main tunnels before exiting the cave, both have multiple Devourers, loot sacks and chests in them. Once you've left the cave, you'll see a rundown building; the initial door you see will be locked, so go around the building to your left to access an open door.  Go inside; as soon as your enter the building, a cutscene will occur with Geralt walking across and seeing many dead bodies, and then a monster runs past into nearby room. Walk into the room to discover some Devourer monsters that you will need to  kill. There are some loot sacks and chests scattered about, not much in them though.  From where you entered; go left and down through the building, exit left through an openning.  Outside, use your Witcher Sense to continue following the trail.

[b]Note: There may be a bug where you do not see Hjalmar's prints leading from the building, you can either try and exit the building where you came, and back track slightly to use witcher senses to pick up the trail again. Or, head directly to the village where Hjalmar should be. The main goal of the quest is to find Hjalmar, so finding him and killing the Giant should complete the quest.[/b]

Once you follow the footprints to the village, you will see Hjalmar fighting some Sirens. After helping Hjalmar kill the Sirens, head towards the cave of the Giant with Hjalmar.

Slay the Giant

Whether you're with Hjalmar alone or you rescued Folan and brought him along, once inside the cave, you'll see the giant sleeping. One of Hjalmar's crew, Vigi, is locked in a cave, and he asks you to release him.

You have a choice to make:

1. Hjalmar suggests you leave him in the cage. If you do that, Vigi will get killed in the ensuing battle. There is no saving him if you don't release him. The battle will end with the survivors looking at the destroyed cage and (very briefly) lamenting Vigi's passing.
2. If you decide to release Vigi, you have to first get the key. The key is over to the left side of the lair. Walk around the patches of snow to avoid waking the giant, use your Witcher Senses to see a chest glowing red: it contains the key. Unlock the cage and Vigi will storm forward and wake the giant.

Vigi waking the giant doesn't really alter the battle, so you may as well take the "good" route and release him. If Vigi doesn't wake the sleeping giant, you'll have to do it yourself by going near him.

The battle with the giant is straightforward: he has two phases. In the first, he tries to bash you with his bare hands. Roll out of the way, attack him with an Ogroid-Oil-enhanced silver sword three times or so, use the Quen sign for backup, then get out of the way of his counter. After a few attacks, he'll grab himself a big anchor as a weapon (which kills Vigi if you didn't free him), and he'll cause more damage if you don't evade quickly enough.

Now, depending on how quickly you take down the giant (remember: Ogroid Oil is your friend), Vigi will survive the battle and will be part of the mission end cutscene.

After killing the giant, you have to decide whether you want Hjalmar to have all the glory or accept his offer to call you the Giant Slayer from now on. Take it, you did the hard work!

After killing the Giant, turn down Hjalmar's offer to leave. Stay behind to loot the giant's corpse and the nearby containers around the lair for some good armor and helpful treasure. The quest will be complete at this point.

You can then head out and fast-travel to the port in the north of the isle to say good bye to Hjalmar and head on to your next quest.