Custom Cards are unofficial Magic cards created by players rather than Wizards of the Coast. They can range from entirely original commanders and mechanics to altered versions of existing cards, with legality determined entirely by the playgroup or environment using them.
Magic's Mystery Booster Playtest Cards, including wonderfully strange designs such as Slivdrazi Monstrosity and Recycla-bird, offer an official glimpse at the experimental design space that custom-card creators frequently explore.
