Grave Strength is a uncommon Sorcery card from Fate Reforged (FRF), collector number 71. Current market price: $0.11 (foil $0.37), TCGplayer market prices via Scryfall.
Choose target creature. Mill three cards, then put a +1/+1 counter on that creature for each creature card in your graveyard. (To mill three cards, put the top three cards of your library into your graveyard.)