Hidden Fates is a special expansion of the Sun & Moon Series of the Pokémon Trading Card Game. In Japan, it was released as GX Ultra Shiny, the twelfth subset in the Pokémon Card Game Sun & Moon Era. It was the second to be considered as an enhancement of the eighth Sun & Moon Era main expansion, Super-Burst Impact.

The core set consisting of cards from the Japanese GX Ultra Shiny and Sky Legend subsets, the Sun & Moon Family Pokémon Card Game set, the Trainer Battle Decks, as well as a number of Japanese SM-P Promotional cards. Hidden Fates also incorporated a Shiny Vault subset, which mostly consisted of Sun & Moon Series reprints featuring Shiny Pokémon. Shiny Vault cards have collection numbers prefixed with "SV". All regular Shiny Pokémon cards featured different artwork to their original prints, while all Shiny Pokémon-GX utilized their prior Full Art print artwork. All have a special embossed Holofoil treatment.

Unlike regular expansions, Hidden Fates booster packs could not be purchased individually from booster boxes. Instead, they were only made available in various set-related merchandise, the first of which was released on August 23, 2019. New merchandise was released periodically throughout the following September, October, and November, offering different quantities of booster packs and complementing promotional cards. Each booster pack contained ten cards, plus one Energy card and a code card for the Pokémon Trading Card Game Online.

The Japanese subset was released on November 2, 2018. Much like preceding High Class Packs The Best of XY and GX Battle Boost, GX Ultra Shiny featured a large amount of reprinted cards. Several of the Pokémon included in the subset featured artwork previously exclusive to promotional cards available outside Japan in various items of merchandise. The subset also includes a large number of Shiny Pokémon, all of which are Secret cards. All regular Shiny Pokémon cards come in a new "S" rarity, while all Shiny Pokémon-GX have a new "SSR" rarity. Other than Pokémon-GX and Prism Star cards, all non-secret cards did not have a rarity symbol. This subset was also the first to introduce secret prints of Stadium cards.

GX Ultra Shiny featured 100 secret cards and had 250 cards in total. At the time of its release, it was the largest Japanese set and the largest set in any language. These accolades were respectively surpassed by the Japanese Shiny Star V subset and the Indonesian Tag Team Collection. GX Ultra Shiny also had a parallel foil set—each card without a rarity symbol featured a Reverse Cracked Ice Holofoil variant. Combined with the standard set, this created a master set of 375 cards. To accommodate the large number of cards, each booster pack contained ten cards instead of the usual five. Each pack contained one guaranteed Pokémon-GX card and two Holofoil cards; a parallel foil card may also be included unless it contains a regular Shiny Pokémon. Each booster box contained an average of two to three Shiny Pokémon, with one guaranteed Shiny Pokémon-GX card.