You need to read a spell called "Orders" from your inventory while standing in front of the blue barrier:
"Speak this spell before the barrier of the Winter King. Its incantations will call the spirits of the Realm of Faery away from their stations, andyou may pass freely."
One of the immculate groups drops this spell. I cant remember which group it was though, there is one by a fire camp near the entrance and there is a big group at the entrance of the elemental forge.
