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After you craft a card, you need to open your deck editor with Q or the deck editor button on the top right. There you can add cards from your vault to your deck. You can also change card variants, which work by making all cards with the same artwork obtain the same effect. If you buy a new card from the merchant or open a new card from a pack that shares the same art as a previous card you already owned, now you can turn all copies into the new more powerful variant making your deck much stronger!