The term 'Mysterious Deck' is a reference to Tarot, a type of card game originating in the 15th century. Tarot is a deck of 78 cards divided into two parts: the Major Arcana and the Minor Arcana. The Major Arcana consists of 22 cards that represent archetypal symbols, while the Minor Arcana consists of 56 cards divided into four suits. Tarot is often used for divination and spiritual guidance, as well as for playing card games.