I think this is a really cool idea, but with the current cost and effect, I'm incline to say that it's a bit on the underpowered side. At least, I wouldn't use it.
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Since there's no rarity below common, what if it did something really nasty with common cards, like replaced them with zero-cost cards with no effect and no keyword (call them "Thin Air" or something)?
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Or, if you weren't to tweak the effect, I would say reduce the cost substantially.
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Edit: You should also probably make it clear whether or not it replaces with cards from the opponent's deck, or just random cards of the appropriate rarity. If it were the latter, that would certainly add an interesting twist.