The first Mysterious card set appeared on June 14, 2018. This set foreshadowed the Steam Summer Sale 2018. The second appeared on December 13, 2018. This set foreshadowed the Steam Awards 2018. Crafting badges from any other game granted one mystery card, replacing coupons you would get regularly. These cards are not craft-able into a badge, but instead will be replaced with a corresponding card once it's sale begins.
Note: If any of the cards in the set that you craft into a badge is blocked from being sold or traded, the crafted card will also be blocked from being sold or traded.