Defense of the Heart is a rare Enchantment card from Urza's Legacy (ULG), collector number 100. Current market price: $21.64 (foil $180.35), TCGplayer market prices via Scryfall.
At the beginning of your upkeep, if an opponent controls three or more creatures, sacrifice this enchantment, search your library for up to two creature cards, put those cards onto the battlefield, then shuffle.