Secret Pack - Yugipedia - Yu-Gi-Oh! wiki

A Secret Pack is a type of card pack in the game Yu-Gi-Oh! Master Duel. These packs are based around a theme, which is typically 1 or more archetypes, and pull from a featured card pool of 40 or fewer cards to help build a Deck based around that theme, in addition to several cards from the Master Pack. Secret Packs serve as the main way to obtain cards for a specific Deck.

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