Six is a rare Legendary Creature — Treefolk card from Modern Horizons 3 (MH3), collector number 430. Current market price: $6.14 (foil $12.65), 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.)