The set's main theme is creature types, and much of the gameplay concerns interactions between these 'tribes'. The story continues the saga of the Mirari from the previous block of expansion sets. The expansion symbol is an artist's rendition of a 'morphed' creature.

Onslaught introduced a number of major mechanics and featured the first return of a keyword from abeyance.

• Creature types - Many effects in Onslaught (Known as "Tribal" effects) depended on a creature's types, i.e. its species, role, etc. These included type-specific creature removal, power boosts, and abilities which grew stronger based on the number of creatures of a certain type which a player controlled.
• Mistform Illusions - A race of creatures who could temporarily change their creature types to any type.
• Cycling - A repeat of the same mechanic from the Urza Block. It lets a player pay a mana cost and discard the Cycling card from his or her hand in order to draw a card. Onslaught expanded on the Cycling mechanic by using more varied mana costs and by including abilities that are triggered by the use of Cycling cards.
• Morph - The most radical new mechanic in Onslaught, this allows players to play creatures as "face-down" creatures with no special abilities, except the ability to turn face-up for a cost. This added an element of guesswork to the game which was greater than any previously present.

Storyline: Kamahl the Barbarian has relocated to the Krosan Forest with the Mirari and becomes a druid. 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 into Phage, a pit fighter whose touch brings death. Phage now allies herself with the Cabal. The illusionist Ixidor, another pit fighter, is wronged by the Cabal. He resolves to avenge the wrong by attacking Phage, helped by the angel Akroma, who is sworn to Ixidor's service. The struggle between Phage and Akroma leads Kamahl into an uneasy alliance with the Cabal against the destructive power of Ixidor and Akroma.