Immediately after finishing the Cove Crafting questline, Magnus gives us this quest. It is simple and to-the-point: rescue Leif! Assuming you've already explored up to the edge of this area, this will be an easy two crowns to nab. Once you do, all of the land in the Pinnacles of Ice section of the kingdom will be open to exploration.
As soon as you complete this quest, Leif himself will give you his first quest, entitled Retracing Frozen Steps.