Demon invasion.

I tried playing a campaign; before this, I had only played separate missions. I completed the first one—despite a total lack of sp in the second half—using an army set + darkness + a cloak that dropped from the Dragon Utopia. In the second one, everything was going well, but around week 3-4, as I approached the second city (the snow town, not counting the one where the 4 heroes start), I saw 2 stacks of 10-12 dragons blocking the path (I needed to break through at least one) and a stack of bears. Was this just bad luck with the random spawn, or is it supposed to be like that?

To be honest, I don't remember such details anymore. If the creature stack isn't composite, then it's most likely random.