Mmm. Nice use of the art work.
By the way, if you want to make the upped unique, I have an alternate art version (one has a mist effect the other doesn't):
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Anyway, I do like the card mechanic. It could be very potent when used with undying kitty. Balancing will depend strongly upon the nature of the crystal creatures available, so its too early to asses at this point I think.