Steam Siege, and the Japanese equivalent Fever-Burst Fighter · Cruel Traitor, are the twelfth and eleventh expansions of the English XY Series and the Japanese XY Era, respectively, and the fourth of XY BREAK. The set continues to feature Mega Evolution as powerful Pokémon-EX and marks the debut of the Steam Pokémon Volcanion in the TCG. It was released in Japan on June 17, 2016, and on August 3, 2016 in English. Steam Siege contains 116 cards, while Fever-Burst Fighter · Cruel Traitor have 59 cards per set, with a total of 113 different cards between both sets.
The expansion reintroduces dual-type cards to the TCG, a mechanic last seen in the Triumphant expansion of the HeartGold & SoulSilver Series. It also features Magearna ahead of its appearance in the Generation VII core games. Many of the illustrations in this set are inspired by steampunk.
Steam Siege has 6 new Pokémon-EX, including 2 Mega Evolution Pokémon, 7 new Pokémon BREAK cards, and 8 dual-type Pokémon cards.