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Yes! I partially retract my words; tonight, in hero mode, I completed this scenario in 82 days (on day 40, I defeated the main hero; in the following five weeks, after eight attempts, I breached the gate with the garrison; and in the last five days, I captured two dwarf cities).
But how else can I improve the result in 40 days? I don't understand.
(And I didn't negotiate with the demon; I kept the "Tome of Chaos" for myself.)
I'm posting the final screenshot (I'm on the bottom line; the rest are the standard saves that come with the game; I don't know how they were made), and I have ten more saves from the game that I can also post.