Forerunner of the Heralds is a uncommon Creature — Merfolk Scout card from Rivals of Ixalan (RIX), collector number 129. Current market price: $0.41 (foil $3.48), TCGplayer market prices via Scryfall.
When this creature enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top. Whenever another Merfolk you control enters, put a +1/+1 counter on this creature.