What is Mystery Booster? It's an awesome experience with the feel of a "Chaos Draft" that uses multiple Magic Draft Boosters from different releases but bundles the cards and variety you'd experience into one amazing package. The set itself contains a whopping 1,670 reprints from across the game's history (see below for the full list), plus 121 copies of something new and surprising—R&D Playtest cards you won't find anywhere outside of Mystery Booster: Convention Edition. There are so many cards packed into the possibilities, you could fill an entire eight-person draft and not see the same card twice.
Cards in Mystery Booster have the original set symbols and numbering of one of their printings. The only difference is a Planeswalker symbol in the lower left corner, like what you find on cards from The List in Set Boosters.
Each Mystery Booster: Convention Edition booster pack contains fifteen cards: fourteen from the gigantically awesome main set, and one from that "something new and surprising" group of R&D Playtest cards.