Magic the Gathering MTG Onslaught Booster box contains 36 booster packs with 15 cards each. Released in October, 2002, Onslaught is the first set in the Onslaught block. The set's main theme is creature types (for example, Clerics and Goblins), and much of the game play 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. The set contains 350 black-bordered cards (110 rare, 110 uncommon, 110 common, and 20 lands). 

The story of Onslaught mainly concerns characters introduced in the Odyssey block. Kamahl the Barbarian 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. Phage then allies herself with the Cabal.

Ixidor, an illusionist, fights in the pits with his partner, a beautiful sorceress named Nivea, who ends up being killed off by Phage. Ixidor then wanders the desert in grief, and discovers his new powers of reality sculpting. Ixidor uses his new power to create a palace in the desert, where he creates an angel based on Nivea named Akroma, who is sworn to his service. He resolves to avenge Nivea's slaying by attacking the Cabal and Phage with Akroma. The escalating struggle between Phage and Akroma leads Kamahl into an uneasy alliance with the Cabal against the destructive power of Ixidor and Akroma.

Onslaught Block marked the end of many traditional elements of Magic: The Gathering. It was the last block to feature the old-style card faces or a storyline set in Dominaria until the release of the Time Spiral block. However, this was not apparent in October 2002 when the set was released.

Onslaught continued a recent trend of increasingly powerful creature based strategies. In particular, Onslaught saw the printing of many aggressively designed Goblin cards. Along with additions from the Legions and Scourge expansions, the speed and consistency of optimized Goblin decks reached a level unmatched by most other creature decks of the time. The archetype has contributed to the ban of several cards in specific formats, including the previously underpowered Goblin Lackey and Goblin Recruiter. It has typically been a competitive deck choice in every format where the Onslaught block is legal.

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 than Urza's Block Cycling cards 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. The mechanic would appear on new cards in the Time Spiral expansion.