Typal Synergy

As you mentioned, the set’s most defining feature became the focus on typal synergies. Initially, Onslaught didn’t have a clear theme beyond a mix of mechanics, but once Mark Rosewater saw that several creatures in the file could change their creature types, he suggested amplifying that concept. This led to the focus on tribal cards — cards that care about the types of creatures you control, like Goblin Sledder, Elvish Promenade, and Skeletal Scrying. It created an entire environment where deckbuilding revolved around picking a tribe and maximizing synergies.

The Morph Mechanic

Morph was another key design decision that rose out of this set, stemming from the Magic Rules Team’s work on older cards like Illusionary Mask and Camouflage. These cards allowed you to play creatures face down without defining them fully, which led to complications with the rules. The fix was to define the properties of face-down creatures, which eventually became the Morph mechanic. This allowed players to play creatures face down for a low cost and then "turn them face-up" later on for their full abilities, providing a strategic layer of bluffing and surprise.

The inclusion of morph opened up a lot of design space for creatures that could change their stats, abilities, or other characteristics based on when they were revealed. Popular cards like Exalted Angel and Darkwater Gargoyle were key examples of how morph created gameplay depth.

Cycling

Cycling, a mechanic originally introduced in Urza's Saga, also made its return in Onslaught, despite initial skepticism. Cycling allowed players to pay a small cost to discard a card in exchange for drawing a new one. Initially thought of as a "recycled" mechanic with limited design space, it turned out that cycling still had plenty of room for innovation. For instance, the card Lightning Rift was an early test card that showed how cycling could be used not just for card advantage but also as a proactive, buildable mechanic that could trigger other abilities.

The return of cycling fit nicely with the set's typal theme, as many tribal synergies worked well with cards that had cycling. This made for a more dynamic and flexible set, where cycling could help fuel other strategies or give players the ability to dig for key creatures that fit their tribal synergies.

The Impact on the Block

The decision to focus on tribal synergies and mechanics like morph and cycling made the Onslaught block — which includes Legions and Scourge — a memorable one for Magic fans. The block pushed the idea of tribal decks to new levels, particularly with the rise of iconic tribes like Goblins, Elves, and Zombies. It also laid the groundwork for future sets to revisit and refine tribal mechanics, especially with the more recent "tribal" blocks like Lorwyn and Ixalan.

Notable Cards

• Akroma, Angel of Wrath: This card became one of Onslaught's most iconic creatures, being part of a cycle of legendary creatures that were excellent examples of high-power, high-impact tribal creatures.
• Gempalm Incinerator: A common Goblin creature with the ability to deal damage to any target creature, illustrating how tribal synergies could be maximized to both clear the board and benefit from creature type.
• Wizards (and cards like Sigarda, Host of Herons): Onslaught also helped establish tribal synergies for Wizards.

In summary, Onslaught’s design history is a clear example of how a set can evolve in response to feedback and the needs of the game. Its focus on typal synergies, the introduction of morph, and the return of cycling helped define the Onslaught block as one of the most influential in Magic: The Gathering's history.