Echoes of the Void

I take the demon city... I move away to my own city, to the distance of the portal to the city... a demon comes out of its second city... I take both demons - the necromancer takes the main one (or the first demon, but rarely)

Added after 32 seconds
And no army in the garrison can handle it. Only the main hero can.

I also tried to block the garrison at first, but after the third attempt, I decided to develop three heroes at once. It's long and tedious, but effective. One hero blocked the warlock and necromancer in the starting castle from below, one blocked the demon garrison, and successfully hired a demon with logistics, who brought reinforcements there. The main necromancer cleared it from above. I understand that it's tedious and takes a long time, but I finished the game.