Kamahl's Druidic Vow is a rare Legendary Sorcery card from Dominaria (DOM), collector number 166. Current market price: $0.81 (foil $3.36), TCGplayer market prices via Scryfall.
(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.) Look at the top X cards of your library. You may put any number of land and/or legendary permanent cards with mana value X or less from among them onto the battlefield. Put the rest into your graveyard.