Arthur, Marigold Knight is a mythic Legendary Creature — Mouse Knight card from Bloomburrow Commander (BLC), collector number 5. Current market price: — (foil $0.38), TCGplayer market prices via Scryfall.
Haste Whenever Arthur and at least one other creature attack, look at the top six cards of your library. You may put a creature card from among them onto the battlefield tapped and attacking. Put the rest on the bottom of your library in a random order. Return that creature to its owner's hand at end of combat.