The following press release reveals the there will be 90 cards in the set (not including secret cards like Alph Lithograph, etc.). It also reveals a major change to theme decks: now they come with a booster pack and a deck box.

Prereleases for this set should be held the weekends of August 7th, 14th, and 21st (due to Worlds being held on the weekend of the 14th, also the 21st is unconfirmed at this point)

No Challenge Too Great, No Battle Too Fierce: HS – Undaunted

The greatest Trainers are those who are fearless in the face of battle. And that’s you! In the Pokémon TCG: HS – Undaunted expansion, you’ll discover battle-ready Pokémon just waiting to be summoned by the fearless Trainer in you! The powerful Rayquaza and Deoxys become a Pokémon LEGEND to be reckoned with… six Pokémon – from Raichu to Umbreon to Slowking – join the ranks of Pokémon Prime… and Stadium cards like the Ruins of Alph and Trainer cards like Legend Box change every player’s strategy. Are you ready to be Undaunted? HS – Undaunted features 90 new cards and remains compatible with previous releases.

* Exciting new Pokémon LEGENDs – Kyogre and Groudon LEGEND and Rayquaza and Deoxys LEGEND!
* Six Pokémon Prime, including Raichu, Slowking, and Umbreon.
* Brand-new Trainer, Supporter, and Stadium cards with new gameplay strategies!
* Features two theme decks built around the Sun Pokémon Espeon (Daybreak) and the Moonlight Pokémon Umbreon (Nightfall).
* Each theme deck now contains a BONUS booster pack and a deck box for storing the deck!
* Holographic parallel set for players and collectors alike.
* 90 cards in all!