Decks are empty when you get them. You have to put cards from your spell list into the deck. Don't forget the treasure cards.
If you're planning on using that deck for a while, you might want to unload the treasure cards from the old deck, so they're available for your new deck.
other than your very first deck, none of the decks you purchase or receive come with any card in them. They are for you to fill with whatever cards you wish, in whatever amounts to wish to fill them (within the limits of the deck -- and of course, you can only puts cards that you have learned in them ).
Like your clothing and equipment, upgrading your decks allows for the growing wizard to expand his/her capabilities in fights.
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