Shadows of the Past is a uncommon Enchantment card from Duel Decks: Nissa vs. Ob Nixilis (DDR), collector number 60. Current market price: $0.35, 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.