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It seems like in the campaign, they said that Nicholas was alive and well. Otherwise, why would we be playing the campaign for the humans? There, Isabel said that she needed to take the hits so that Nicholas could move forward... but I never understood where he was going.
He was killed at the end of the human campaign. In the Inferno campaign, Argyle took the amulet, the Griffin's Heart, from his corpse.
Nicholas was supposed to move towards Isabel so that she could provide him with reinforcements, or in other cases, Isabel simply distracted the demons so that Nicholas could defeat the fragmented forces.