Yorion is simple, when it enters the battlefield, exile any number of nonland permanents you control until the next end step
This can be very strong on its own because it means that opposing players will have to discard some of their cards if Kefka is left unattended, and the user can discard valuable combo pieces in order to later retrieve them with the Commander staple called Underworld Breach, particularly if comboed with Lion's Eye Diamond to create infinite mana and then win the game
Singleton deck building As explained above, you can only have one of each card type in your deck, except for basic lands
All the various choices made from worldbuilding to names to flavor text to art to card mechanics blended seamlessly to make a truly amazing new world, one that both explored things Magic had never done, yet also felt at its core, like something the game could have done thirty years ago
Im very happy to see a response-heavy blue-black deck that leans on a creature type havent really seen often before