The objective is to complete all 26 skill cards in your deck, which will grant you a large experience reward in any skill, with the amount dependent on your level, in the same way as Treasure Hunter lamps. You can only win prismatic skill cards from 29 August until 9 September, but you can continue to complete your previously won cards until 9 October. If you don't complete all of the cards, you can turn your incomplete deck in for a substantially smaller experience reward.