Hjalmar is part of the quest A Hunting We Shall Go. You must find him and his orb bodied self and kill him. His location is in the map on the right. When you encounter Hjalmar, whether you're doing his related quest or not, he will say "Totems be praised, we have our sacrifice. Prepare to burn and feast, goblins of the valley!" He and his goblins will then proceed to attack you. Once you kill him, you must loot Hjalmar's mask from his corpse.