Oath of Lieges is a rare Enchantment card from Exodus (EXO), collector number 11. Current market price: $3.99, TCGplayer market prices via Scryfall.
At the beginning of each player's upkeep, that player chooses target player who controls more lands than they do and is their opponent. The first player may search their library for a basic land card, put that card onto the battlefield, then shuffle.