Uninvited guests.

I completed the game as Haven. Combat tactics: Monks + Vampirism (obtained near the witches' coven). After acquiring Subjugation and Berserk from the Necromancer, everything became easy. As for the Orcs, the hero turned out to be super powerful. Apparently, an incredibly lucky combination of circumstances can help you win. Dark and Chaos magic is annoying for opponents. If you invest in Dark and Chaos Suppression, then without fully maxed Attack and Machine Control, the enemy's living forces are defeated; if you invest the other way around, then magic is defeated (I liked the Warlock's explosion, which destroyed 100 centaurs at once) by the middle of the second month.