The first time Pokemon TCG Pocket introduced a mechanic like this was with the release of the Extradimensional Crisis pack
For the Unknown Event cards, your best bet is probably to proxy any that have previously existed, though you might occasionally find a few on the secondary market
This ability is so powerful because it lets you enchant your creatures at least once and get value out of your auras
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Gold etched stock scratches and shows edge wear readily, so inspect the surface hard before submitting and reserve it for copies with 55/45 or better centering on both axes
