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Yes, I agree that the question was not formulated clearly enough, but any clarifications would lead to an obvious answer.
The correct answer: *the creature I had in mind is a mythical dragon.* I am currently playing a map where I already have quite a few of these creatures in my army. And I noticed that the damage inflicted by their magic ranges from 0 to the maximum. Therefore, it is impossible to express the spread in percentages, but it is obviously greater than that of any other creature.
*Zero damage* is possible when the dragons are under the control of a hero and they cast Ice Ring on a 1-hex stack, without causing any harm. Therefore, the player should first see what spell the dragons have prepared (it is written in the battle log at the bottom of the screen), and then carefully indicate the target for the attack.
*The maximum damage*, on the other hand, depends on many nuances: the spell, the arrangement of the stacks, immunity/resistance to magic, etc.