Location: Undvik (Skellige), then Kaer Morhen
Don’t forget about these two lunatics! Assuming you met and helped them, both Folan and Vigi provide an almost animal-like ferocity to the forthcoming battle, and will already have made the journey to Kaer Morhen (so you don’t invite them personally during this quest; just expect them to accompany Hjalmar when you invite him). To ensure they help out, you must complete the following:
Secondary Quest: The Lord of Undvik saved Folan from the rock troll’s cauldron, and saved Vigi from his prison during the battle with the mountain giant.