Майор Вихрь
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The Shadow creatures have less health (105 versus 120), and their stats seem to indicate a slightly better attack, with the same defense. And consuming corpses is useful against neutral creatures; a couple of Pack Animals die, they eat the corpses, and there are no losses. (And regeneration of 30-50, as you yourself said, isn't very useful).
I'm not even talking about the fact that it's not certain that this unit will sustain damage for all three turns; it might be killed earlier. And if it's some kind of large and slow unit, like a Hydra or an Ent, then the poison is generally not very effective. But for lower levels, it can be more or less useful; however, again, the attack of such a large number of Wyverns can simply tear them to shreds without any poison. Plus, not all creatures are affected by it.
I'm not even talking about the fact that it's not certain that this unit will sustain damage for all three turns; it might be killed earlier. And if it's some kind of large and slow unit, like a Hydra or an Ent, then the poison is generally not very effective. But for lower levels, it can be more or less useful; however, again, the attack of such a large number of Wyverns can simply tear them to shreds without any poison. Plus, not all creatures are affected by it.