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You probably mean the first turn of the Knights. Of course, there might be no one in their path. But, for example, there are demons, nightmares, and cerberuses that can be targeted in this way. And no one is forcing you to send them against the enemy at the very beginning. When his army is a little more spread out, it becomes quite feasible to target more than one. And the Knights will tear through demons with their gate ability. But if they dealt full damage with this ability, it would be an awful imbalance! No ignoring defense would compare to that.