Six is a rare Legendary Creature — Treefolk card from Modern Horizons 3 Promos (PMH3), collector number 169p. Current market price: $5.02 (foil $5.88), TCGplayer market prices via Scryfall.
Reach Whenever Six attacks, mill three cards. You may put a land card from among them into your hand. During your turn, nonland permanent cards in your graveyard have retrace. (You may cast permanent cards from your graveyard by discarding a land card in addition to paying their other costs.)