Blood Oath is a rare Instant card from Mercadian Masques (MMQ), collector number 177. Current market price: $1.39 (foil $4.75), TCGplayer market prices via Scryfall.
Choose a card type. Target opponent reveals their hand. Blood Oath deals 3 damage to that player for each card of the chosen type revealed this way. (Artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery are card types.)