Ment
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I think Hafada isn't necessary in that case.
Or, we could try to rethink similar abilities to make them easier to implement. For example, instead of killing a percentage of units, we could reduce the health of all units by a percentage (I'm not sure if this idea would work, but it demonstrates my line of thinking).
Or, we could actually make summoned units attack the enemy army, but without calculating percentages of units to kill, and instead use different rules.