You need to make your way to Glacial Cave in the northern part of the map and then after killing the three Rieklings inside, pick up the [ Karstaag's Skull ] from the ice wall at the back. Now it's time to head to Karstaag's throne of which there's two ways to reach it.
The first route requires you to head to Castle Karstaag Ruins and pick the master-level lock here (so save right before you attempt it, giving you infinite attempts). The second - and longer - route is through the Castle Kaarstag Caverns .
It's jammed packed with Rieklings, so take your time (and you can also complete the other side-mission Recover the Bonemold Formula for Glover Mallory whilst you're down here). You'll soon reach the throne room (where the picked lock option takes you). Place the mask on the skeleton of Kaartag and then move away as the boss battle will begin!
It's worth noting straight away that this boss will deal very high levels of ice damage to you and it is itself resistant to ice, poison and disease attacks (so don't even bother trying). It's a good idea to leave this quest until near the end as you will ideally want to be armed with loads of Black Book spells.
Companions will generally go down quickly too, so you'll want to be packed up with loads of ice resist potions. Fire mages can do well, as too can those with very high sneak attributes.
Once the boss is defeated, you'll recieve the power to summon Kaarstag for a total of three times . He'll join you in any outdoor battle you require for 120 seconds (and will likely make it an insta-win too), but once you've called him for the third time, you can't call him anymore.