That's a neat card. One use for it that nobody mentioned yet would be to play on mutants. The mutant would revert to what the actual card says, rather than whatever random mutation it got. I know when I used to play scaredgirl's beginner FG grinder that I would on occasion use Eternity on a mutant that had a really good creature as its base, like maybe an Amber Nymph with devour, for example, that is far more valuable in its original form.
I think it's costed appropriately. It's kind of like a lobotomize or rewind, but not really. It's also in most cases weaker than the in-element antimatter, so it should definitely be cheaper than that.
The uses I see for this card:
-change mutants to whatever the printed card says
-shrink your opponent's pumped creatures. (that 30/24 lava destroyer? sorry, it's 7/1 again. that 5/10 otyugh? sorry, 0/5 now)
-healing a creature and/or curing its poison/aflatoxin
-unlobotomizing something by giving back an ability it lost somehow
-I suppose it could rebirth a phoenix, but that's a waste
-undoing antimatter
Haha, one question I just thought of, what happens if you play this on a Pharaoh or creature that used to be a Mummy or Skeleton, but got rewound?