Trostani, Selesnya's Voice is a mythic Legendary Creature — Dryad card from Return to Ravnica (RTR), collector number 206. Current market price: $2.15 (foil $19.42), TCGplayer market prices via Scryfall.
Whenever another creature you control enters, you gain life equal to that creature's toughness. {1}{G}{W}, {T}: Populate. (Create a token that's a copy of a creature token you control.)