I'm in the fifth mission of the first campaign. As an experiment, I broke through the garrison, took the demon castle, and destroyed the Agril, but it didn't change anything. Nicholas reached the place where the Agril used to attack him, and then a cutscene played where he was killed, and the mission ended.
Added 5 minutes later
If I had taken Wolfsten's army and/or arrived with a full mana supply, I would have definitely defeated him.