Ascended Heroes is themed around the video game Pokémon Legends: Z-A, and features various new cards based on Pokémon and characters introduced in the game. It also introduces new and returning Mega Evolution Pokémon ex and Trainer's Pokémon cards with new illustrations. This is the first international set to introduce cards with the  regulation mark to the Standard format of the Pokémon Trading Card Game.

The Mega attack rare rarity is a new type of card rarity introduced in the Ascended Heroes expansion, originally released in the Japanese MEGA Dream ex subset. Mega attack rare cards are all Full Art Mega Evolution Pokémon ex with artworks rendered in a pop art style, and the card's highlight attack written with large text in the foreground. The attack text differs between the Japanese and English localizations; the English card art has Japanese text for the card's attack and vice versa. This stylization is reminiscent of Mega Evolution Pokémon-EX cards released during the XY Series.

Ascended Heroes includes cards from the Japanese MEGA Dream ex subset and the Start Deck 100 Battle Collection. Ascended Heroes contains 13 Mega Evolution Pokémon ex; six Tera Pokémon ex and 20 Pokémon ex; 33 illustration rare Pokémon; 14 ultra rare Mega Evolution Pokémon ex, Pokémon ex and Trainer cards; seven Mega attack rare Mega Evolution Pokémon ex; and 22 special illustration rare Mega Evolution Pokémon ex, Pokémon ex and Supporter cards. Ascended Heroes is available in English, German, French, Italian, Spanish, and Portuguese. With 295 cards, Ascended Heroes is the largest Pokémon TCG expansion in the aforementioned languages, beating the previous record of 284 cards held by the Fusion Strike expansion (although Fusion Strike has more non-Secret cards).

Like MEGA Dream ex, this special expansion includes two parallel Holofoil variants that may appear on low-rarity Pokémon cards, featuring either a type of Poké Ball or an Energy symbol matching the type of the card. Pokémon cards of common, uncommon, and rare rarity are available in both variants, without a regular reverse pattern. The Poké Balls that can appear are the Love Ball, Friend Ball, Quick Ball, Dusk Ball, and standard Poké Ball; and all Team Rocket's Pokémon use the Team Rocket "R" instead. Each Pokémon has one specific type of Poké Ball pattern, and each Evolved Pokémon has the same pattern as their pre-Evolved version. All first partner Pokémon, including the Pikachu family, are among the cards that use Friend Balls. All Mythical Pokémon and non-Team Rocket Trainer's Pokémon use standard Poké Balls, and they are the only cards to do so. Trainer and Special Energy cards of common and uncommon rarity are available in the regular Mega Evolution Series reverse pattern only.