Asthohtsa, Who Dreams

Flying, menace
Whenever Asthohtsa enters the battlefield,
exile target nonland permanent an opponent
controls if its converted mana cost is less than
the mana spent to cast it. Repeat this process for
each time Asthohtsa has been cast from the
Command Zone.
Asthohtsa gets +X/+X, where X is the greatest
converted mana cost among nonland permanents exiled with Asthohtsa's effect.