How to unlock: Complete the On Ameridan's Trail quest.
Quest giver: Svara Sun-Hair (M31,10)
Note - before beginning this quest, it is best to first complete all the other quests and activities in this location. After triggering this mission, darkness will cover the whole location and only an aurora will be giving some light. If you decide to take up some new tasks after this quest, it may take you more time to finish them.
When you start this mission, you will go to the northern gate (M31,17a) with Avvars. Keep eliminating enemies under the gate until it's open. Behind it, you will find some more opponents and a way down, with three assassins on it. At the bottom, you will be attacked by a few more enemies.
You will notice small orbs that generate cold and they may cause you some trouble. Destroy them first. Locate the door to the ruins (M31.17b) and go inside.
When in the ruins, you must keep an eye on the cold bar. When it reaches the end, you and your whole team will start losing hp. You can regenerate the bar near bonfires. Some of them require to be activated. Head straight through the ruins or turn right on the first fork, where you will find veil fire torches to activate (M32,1a). It will allow you to examine several runes (M32,[1b,1c,1d,1e]) that hide interesting blueprints from one set.
When the path forks for the second time, turn right. At the end of the corridor, you will find a key (M32,1f). It is required to open another door (M32,1g).
Go down the stairs and follow the path until you reach a large hall. Inside, you will face a boss - Gurd Harofsen (M32,1h). First, look around the arena, because you need to locate bonfires. The enemy will be extinguishing the nearby ones with his breath. Before starting to attack the boss, eliminate all Jaws of Hakkon. If you're a melee fighter, watch out for the cold breaths. Try to dodge them, because these attacks can freeze you. When the fight is over, you will watch a cut-scene and receive another task. You will also have some time to collect the loot. The cold will not bother you from now - it will leave the place along with the dragon, whom you will be fighting in a while.