No Witnesses is a rare Sorcery card from Murders at Karlov Manor (MKM), collector number 27. Current market price: $0.22 (foil $0.33), TCGplayer market prices via Scryfall.
Each player who controls the most creatures investigates. Then destroy all creatures. (To investigate, create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")