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If you complete the optional quest in the demonic underground in the second mission and sacrifice the Orcs, they will revolt, blame the mages, and later come to beat them up, along with everyone else in the area. But they only spawn when the quest is completed, as an additional feature.
A strange optional quest; I didn't have anything like that happen, only the option to attack the mage in the underground and close the portal. It seems something broke in the script, and nothing was triggered.
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I spent an hour with the mentor, leveling up abilities, made Sandro a farmer with resource gathering, leveled up Markel into a ghost in the mines with support, and the dummy purely into chaos and defense so she could defend the castle, and now the fourth mission is starting, and I don't know what to do.))
The third mission is also easy; it was very difficult at the beginning due to the lack of mana fountains, but once Lucretia reached the location, 20 vampires arrived and solved everything.
The mages didn't leave the castle at all after the Orc appeared, and then the Orc took over the mages' eastern-southern castle and stayed there for the rest of the game. Somewhere around the 3-month mark, everyone went AFK.