Herald's Horn is a uncommon Artifact card from Modern Horizons 3 Commander (M3C), collector number 296. Current market price: $5.89, TCGplayer market prices via Scryfall.
As this artifact enters, choose a creature type. Creature spells you cast of the chosen type cost {1} less to cast. At the beginning of your upkeep, look at the top card of your library. If it's a creature card of the chosen type, you may reveal it and put it into your hand.