Hallowed Fountain is a rare Land — Plains Island card from Ravnica Allegiance Promos (PRNA), collector number 251p. Current market price: $12.26 (foil $11.95), TCGplayer market prices via Scryfall.
({T}: Add {W} or {U}.) As this land enters, you may pay 2 life. If you don't, it enters tapped.