Mystery Booster is a special Magic: The Gathering set designed as a "Chaos Draft" experience, where cards from all across the history of Magic can show up at random. The set contains nearly 2,000 different cards, making it the largest expansion to date.
In its first print run, Mystery Booster was exclusive to conventions, and featured 121 weird, wacky "playtest cards" that were usable only in draft and sealed formats. The following year, the set was made available in retail, with the playtest cards replaced with 121 foil reprints. The year after that, a new printing of the Convention Edition was distributed to affiliated local game stores to celebrate the return of in-store play after the lifting of pandemic restrictions.
Unlike most Magic expansions, Mystery Booster was only printed in English. It was sold in draft boosters in displays of 24 per box.