Mupy-Mup!
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And besides, banishment itself is quite a weak ability.
Yes, it's rather weak, especially if only used at the very end when you don't need much power, but rather massiveness, i.e., speed. While we're slowly eliminating a few units at a time, this "mademoiselle" will waste a lot of our time. But it makes sense that it should reduce the summoning cost. Of course, the chance of someone else finding it on the map is very low, so it probably doesn't make sense to change it just for one instance.
I just thought when I saw it that the author had created some new feature, and there were hints of changes related to Necromancers above.