I really love the idea, but the upgraded version should still have 1 hp. Perhaps less expensive upgraded version ? (and a bit less power)
I like it because it is like "i'm gonna kill your creatures, and if i kill em all you gonna die to my awesome monster"
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You could even make it be "if there is a non airborne creature on the field, this creature doesn't attack" Because it shouldn't fear birds from deep water. (just a little idea to try to create some use to airborne passive by the way
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