Demon invasion.

I'm actually replaying the campaign right now on Hero difficulty (I had one playthrough on Veteran). I completed the first mission without any major problems; the underground section can actually be completed almost regardless of the circumstances. You just sometimes defeat the computer-controlled enemies, and sometimes you just run around, trying to make them fight each other. The only downside is the lack of any motivation to complete the underground section quickly, because the neutral creatures on the surface are initially large and don't grow, meaning it's more beneficial to spend more time collecting upgrades (after the transfer, my stats were 4-1-6-3), so it takes 3-4 months to complete the underground section, which is a long time. But the surface is completed almost without problems once you know what the enemies are like; I only needed 2 attempts for Hangwul and Wolfenstein, so I spent a total of 12 months on the mission, which is much faster than one playthrough. In the second mission, I was simply amazed by the Necromancer's army on Hero difficulty (which is 2.5-3 times larger than on Veteran). I defeated him with great difficulty after several hours of attempts, and that was with a fairly good starting position (11 spells + Trident) . I hope the Necromancer is one of those who got nerfed)). But everything went very well on the surface. I developed Nimue, and her special ability + building Chaos Gates in all castles = 100% Chaos Gates by the time the first Knight arrives. This allowed me to keep all 5 castles. Sigrif also had a good start; I had a starting hero with a Mage skill, and I learned all the high-level Dark magic spells from the demons in Shi'o while they were allies. After the ritual, I accumulated some troops + summoned reinforcements from the surface and easily defeated both strong demons in the Dark castle. I didn't accumulate troops for Kha-Beleth for long; I defeated him mainly thanks to the powerful Chaos Gates (8 demonic castles!!!) . And in the final battle on the surface, Faydaen again gave all his good troops to Duncan (including his own archers, which made his special ability useless), and the battle with Duncan was very difficult , but Faydaen himself was defeated without problems. In total, it took 9 months and 2 weeks to complete the mission; I think it can be done even faster. In the third mission, so far, the only good starting position was ruined by a week of doubled growth, which made it impossible to rush the Brown hero. Of course, there can also be very unpleasant neutral creatures that can be randomly generated, but it's tolerable; however, the random nature of the skill progression is unforgiving. I think it's necessary to guarantee protection in the Witch Hut. And I don't know if this AI behavior will repeat, but last time:

UPD. I think I figured out the reason for Grok's stupidity in the underground section. He either doesn't have the Ship Summon spell, or he doesn't want to use it for some reason. I just checked - I brought a secondary hero's ship to the exit to the surface, where there were Emerald Dragons, and he immediately ran there and sailed to capture my castle.

In general, I'll write more about this later