Behind the Mask is a common Instant card from Murders at Karlov Manor (MKM), collector number 39. Current market price: $0.20 (foil $0.11), TCGplayer market prices via Scryfall.
As an additional cost to cast this spell, you may collect evidence 6. (Exile cards with total mana value 6 or greater from your graveyard.) Until end of turn, target artifact or creature becomes an artifact creature with base power and toughness 4/3. If evidence was collected, it has base power and toughness 1/1 until end of turn instead.