As a little clarification:
Go to the above coordinates, and you'll find yourself in a cave. There are some hostile mobs there; these are not what you want. Keep going down the cave and you'll find Slumbering Mjordin(Neutral). You will want to interact with these like any other NPC. They will have a dialogue option and when you click it they will either turn in to a normal hostile target which you must kill, or Iskalder.
Iskalder is an elite mob, and when you find him you'll get a raid warning saying something along the lines of "You have found him! Now is the time to use the The Bone Witch's Amulet!" He is an elite, but he wont do too much damage to you before the cast goes off.
After this you have to return to the bone witch. 2 helpful (I hope) facts:
1. He will not assist you in combat.
2. He can despawn, but it isn't as easy as I believed from other comments. I got on my flyer and just ran on the ground until something agro'd and then flew up to despawn.
Maybe too much information for a simple quest, but I was a bit confused and that's what this site is for, right?
Edit: map.