A continuation of the storyline developed in the Odyssey Cycle, Onslaught is a set on the continent of Otaria on the devastated plane of Dominaria, approximately a century after the events of Apocalypse of the Invasion block. Kamahl has relocated to the Krosan Forest with the Mirari and becomes a druid of Krosa. The power of the Mirari begins to work on the forest, mutating its residents and growing some of them to gigantic proportions. Meanwhile, Kamahl's sister Jeska is transformed by foul Cabal healing magic, into Phage, a pit fighter whose touch brings death. The illusionist Ixidor discovers his new powers of reality sculpting. Ixidor uses his new power to create the angel Akroma.

Each Onslaught Tournament Deck box comes with 12 decks. Each deck contains 75 mint collectible trading cards including 2 rares, 10 uncommon, 32 common, 30 lands, and a rulebook.