Ull's Sanctum
When you reach the map marker, you will find Ull's cave,
Enter it and search for Ull. There will be some enemies littered throughout the cave so deal with them by chucking spears at them.
Eventually you will reach the colder parts of the cave that ice shards appear on the roof. The game tells you to shoot them. There even is a glowing area under them to show the area that will get hit. This will be useful for the boss fight later since you will use this to deal substantial amounts of damage to Ull.
When you reach Ull's main area, you can get a quick and easy, free hit on him by dropping ice shards on him and his men. Do a takedown behind him when Ull has been stunned by the ice shards.
There will also be numerous amounts of Udam constantly spawning in the battle. Keep moving and they will have trouble hitting you. Continue putting the pressure on Ull.
When you drop Ull's HP to none, shoot an ice shard to make it come down on him to finish him. Just as you are about the finish though, a Cry will be heard. Ull then goes towards the sound.
You will find Ull's children. In his desperation, he threatens to kill the baby but soon gives up. He asks you to protect his children since the Udam are slowly dying.
After Ull passes, the mission is now complete and you will report the success to your people.