Shadows of the Past is a uncommon Enchantment card from Magic Origins (ORI), collector number 118. Current market price: $0.15 (foil $0.86), TCGplayer market prices via Scryfall.
Whenever a creature dies, scry 1. (Look at the top card of your library. You may put that card on the bottom.) {4}{B}: Each opponent loses 2 life and you gain 2 life. Activate only if there are four or more creature cards in your graveyard.