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It’s neither a bug nor a consequence of using cheats. It’s a consequence of not taking into account this line in the topic header: “The map is a clean mod! A clean version of the game is recommended!”. This is a very common situation. You can find mentions of it in the comments several times.
These demons are from my own map, “Shades of Darkness,” which is best not to be played simultaneously with “The Path of the Necromancer” and “The Price of Loyalty.”
By the way, your brother seems to know the game quite well, judging by the tricks he’s pulling.