Case of the Burning Masks is a uncommon Enchantment — Case card from Murders at Karlov Manor (MKM), collector number 113. Current market price: $0.10 (foil $0.07), TCGplayer market prices via Scryfall.
When this Case enters, it deals 3 damage to target creature an opponent controls. To solve — Three or more sources you controlled dealt damage this turn. (If unsolved, solve at the beginning of your end step.) Solved — Sacrifice this Case: Exile the top three cards of your library. Choose one of them. You may play that card this turn.