Based on your description, I can't understand which map you're talking about, but I probably haven't played it: I don't remember such a victory condition; everywhere, it required defeating all opponents.
(Василь Симоненко. Люди – прекрасні)
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Strange that your approach worked: Madre has all spells (checked in the editor), would she really allow herself to be blinded and not remove the blindness? As far as I know from experience, the AI always tries to remove blindness. Even when the enemy hero has no healing, no dispel, and no anti-magic, he will definitely cast something (Chain Lightning, Armageddon), as a result of which blindness will be removed.
And haven't you tried to get Berserk? Maybe there is still some way?
Anti-Magic is needed to protect against Faerie Dragons in general, both their magic and the reflection of our spells — Liches cannot be blinded (but Forgetfulness can be cast on them).
On the contrary, I regularly observe that the AI doesn't care about blindness.
And I didn't see the Executioner spell in the book, so I still don't like it. After all, no matter how good you are as a player, the factor of luck/bad luck also sometimes plays a significant role.
Well, I don't know... maybe we have different "Heroes": :) in my game, the AI is very resourceful and almost always senses trouble when I cast it.
The fact that almost nothing is challenging for experienced players pushes some authors to use completely mind-blowing techniques.
The other day, I looked at the map "Qin the supreme" because I saw its discussion on Maps4Heroes. It turned out that my old joke about "defeating 100 dragons with one pikeman" is no longer a joke. A hero of level 6424 with 4 peasants attacks a town and... wins! The wave of death he unleashes completely destroys the garrison in the town, but 1 peasant remains alive (the other 3 were killed by the tower shots)!