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There's a bug: as soon as a hero casts "Evasion," the opponent flees with an army that has every chance of winning the battle. The same happens if a dwarven composite artifact is present, which automatically casts "Evasion" before the battle – without inflicting damage, the AI surrenders with any army size.

Almost everything is correct. The general "Evasion" spell has a magical property for the AI – it causes them to flee. However, with a full set of dwarven artifacts, the battle does proceed.

Dark magic is learned by developing "Keen Eye" from the "Education" skill line.

You are right. But it's important to know the map structure, as there won't be a normal place to learn spells. By the time it becomes obvious, the skill stacks are already filled. (I, for example, don't like this "Keen Eye"), retraining just for this is like scratching your right ear with your left foot.