Descendants' Path is a rare Enchantment card from Avacyn Restored (AVR), collector number 173. Current market price: $3.05 (foil $15.45), TCGplayer market prices via Scryfall.
At the beginning of your upkeep, reveal the top card of your library. If it's a creature card that shares a creature type with a creature you control, you may cast it without paying its mana cost. If you don't cast it, put it on the bottom of your library.