Herald's Horn is a uncommon Artifact card from March of the Machine Commander (MOC), collector number 360. Current market price: $6.31, 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.