The Pokémon TCG: XY Steam Siege expansion set was announced by the Pokémon Company on June 8th, 2016 and it was released on August 3rd, 2016. Images from the press release have been added to an Imageboard thread.

Press Release - Pokémon Volcanion and Magearna make their Steam Seige Trading Card Game debut Pokémon Trading Card Game

The Pokémon Trading Card Game (TCG) is a collectible and competitive trading card game. In the competitive Pokémon TCG, each player constructs a deck of 60 cards and uses it to battle against other players in fast-paced, strategy-oriented play. Since the debut of the Pokémon TCG in 1996, more than 21.5 billion cards have been shipped to 74 countries around the world.

Pokémon TCG: XY Steam Siege

Pokémon TCG: XY Steam Siege is the latest expansion to the Pokémon Trading Card Game franchise, featuring the powerful BREAK Evolution mechanic, giving evolved Pokémon even more power during battle! When Shiny Mega Gardevoir-EX and a host of greedy forces lay siege to the mighty Gear Palace, the Mythical Steam Pokémon Volcanion must wield the dual powers of fire and water to stop them! As the battle rages, Xerneas BREAK and Yveltal BREAK face off in this exciting new expansion. Booster packs and two theme decks—Gears of Fire and Ring of Lightning—will be available in stores on August 3, 2016, introducing more than 115 new cards to collect and play.

Key Features of the Pokémon TCG: XY—Steam Siege expansion

• Xerneas BREAK and Yveltal BREAK debut— The popular Legendary Pokémon Xerneas and Yveltal appear as Pokémon BREAK, giving players added strength in battle.
• Seven new Pokémon BREAK— Five additional Pokémon BREAK join Xerneas BREAK and Yveltal BREAK. These new cards, featuring landscape orientation, a unique foil pattern, and “gold-plated” art rendering, include Yanmega BREAK, Talonflame BREAK, Pyroar BREAK, Clawitzer BREAK, and Hydreigon BREAK. These cards give the evolved Pokémon more power, adding an HP boost and an extra attack or Ability to the Pokémon’s existing attacks (similar to the LV. X mechanic from the Diamond & Pearl Series).
• Volcanion and Magearna make their TCG debut— For the first time, the Mythical Pokémon Volcanion and Magearna make an appearance in the Pokémon TCG, and do so in a very big way. The expansion features Volcanion as both a regular card and a dual-type Pokémon-EX, while Magearna-EX makes its impactful debut.
• Dual-type Pokémon return— For the first time since the Pokémon TCG: HeartGold & SoulSilver Series, dual-type Pokémon make an appearance in XY—Steam Siege. Dual-type Pokémon give more options to players in deck building, and the new expansion includes three dual-type Pokémon -EX, including two Mega Evolution Pokémon, to add to your deck.
• More Mega Evolution Pokémon and Pokémon-EX— Mega Steelix-EX makes its Pokémon TCG debut, and a new Shiny Mega Gardevoir-EX is also featured in the new expansion. These incredibly powerful cards are joined by four more new Pokémon-EX: Steelix-EX, Magearna-EX, Gardevoir-EX, and Volcanion-EX.
• Booster packs—Packs of 10 additional cards selected from the expansion.
• Two new theme decks—These 60-card preconstructed decks are designed to introduce players to the Pokémon TCG by enabling them to play right away.

Pokémon TCG: XY Steam Siege Elite Trainer Box

The Pokémon TCG: XY Steam Siege Elite Trainer Box was released on August 3rd, 2016. Wield the volatile powers of fire and water in your next game with the mighty Pokémon TCG: XY—Steam Siege Elite Trainer Box! Inside, you’ll find everything you need for your next major Pokémon TCG battle, including card sleeves, high-quality dice and condition markers, extra Energy cards, and a strategy guide to the XY—Steam Siege expansion. Reign supreme in your next battles with the Pokémon TCG: XY—Steam Siege Elite Trainer Box!


Join the Elite Trainers with the TCG: XY—Steam Siege Elite Trainer Box! Inside, you’ll find:

• 8 Pokémon TCG: XY—Steam Siege booster packs
• 65 card sleeves featuring Volcanion
• 45 Pokémon TCG Energy cards
• A player’s guide to the XY—Steam Siege expansion
• 6 damage-counter dice
• 1 competition-legal coin-flip die
• 2 acrylic condition markers
• A collector’s box to hold everything, with 4 dividers to keep it organized
• A code card for the Pokémon Trading Card Game Online

Please note that the player’s guide explanation of the Captivating Poké Puff card contains an error. If your opponent’s Pokémon has an Ability that can be used when it comes into play, that Ability does not work if you use a Captivating Poké Puff to put it onto your opponent’s Bench.

Source: https://www.pokemon.com/us/pokemon-tcg/product-gallery/xy-steam-siege-elite-trainer-box