Demon invasion.

Nibros was tailing me, so I only cleared 3 zones with the elevation bonuses. I managed it pretty quickly, but I found 400 decaying zombies guarding a castle in the second part (I took them down easily, they were just zombies). I also had a horde of astral dragons, 300 gremlins, 200 fire demons, 44 death knights, etc. I killed everyone except the dragons. And from the mines, I only took the sawmill. One mine had 150 of each type of assassin, and the other had a bunch of fire elementals. That's how it went. The castle has everything up to succubi, but the last ones and the hounds haven't been upgraded.

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Found spellbook 4 + an artifact.

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And in the second part, the hero started with just 1 cave lurker.