"In Honor of Our Fathers"; - "Wisdom of the Earth" [Campaign 3, Mission 4]

how do I complete this mission? I first gathered an army, opened passages to the tents, and went through the prophets, but at the end, I encountered a demon who is much stronger than me, and I don't understand what to do. I tried simply gathering an army and not paying attention to the tents; I found a fortress, improved my army, but at the end, it's still the same demon, and he's still stronger than me. The question is, what should I do and how should I proceed?

Added after 41 seconds
the demon killed me* when I went through the prophets' portals, everything was fine.**