Throughout Magic: The Gathering's history, there has always been a scale of rarity in the mix. There are the basic lands at the bottom, and other cards quickly scale up to common, uncommon, rare, and mythic rare. Other categories, such as Timeshifted and Masterpiece, have also been released over the years, creating quite the spectrum of card strength and how often they can appear in a booster pack.
For several years, Magic really hasn't created a new rarity level, the last being Masterpiece during Zendikar Expeditions in 2015. But then, in November 2020, something unusual happened. (You know, outside of American politics and global health crises). A new rarity emerged.
Unlike previous new rarities and categories, it wasn't really advertised so much as it just appeared, catching a lot of people off guard. When a cereal like Lucky Charms decides to add a new marshmallow, or when Mars, Inc. decides to recolor one of their M&M's, there is usually an advertising blitz to 1) get more people excited about it and 2) not freak people out with sudden new changes. For Magic, a new rarity is usually seen as special, but thanks to the overall state of the world in 2020, there wasn't a lot of time to really energize people about this. Still, even though it didn't arrive with a lot of fanfare, this new rarity was special.
So what did they name this one? Oh. Special. That's like naming the orange colored Starburst "Orange", now isn't it?
For the most part, special cards are simply common cards, but they act like commanders and basic lands in that they are always available. Oh, and they have an "S" instead of a "C" by the card number.
How many of these new cards have been made to date? So far... one. One single Special card. The Special designation was retroactively added to a few prior cards, but they are considered to be above mythic rare. Only one purely Special card has been printed: The Prismatic Piper. It was so odd that Magic developers had to keep making statements on how to play it. Essentially, it was a card created to cover a lot of different cases, such as failing to draft a commander card in Commander, or having no specific colored mana needed, which ultimately resulted in it being deemed "special". And it hasn't made a comeback since. It's like when Magic introduces a creature type or future so odd or specific that they don't know what to do with it. It's a card so esoteric and specific that making variations on it can actually take a bit of work.
There will undoubtedly be more Special cards in the future, but for now The Prismatic Piper is in a class all by it's own. There are seas of commons, scores of uncommons, and several mythic rares. But for now, there is only one that is truly Special.