A Mysterious Creature is a common Creature card from Murders at Karlov Manor Tokens (TMKM), collector number 21. Current market price: $0.28, TCGplayer market prices via Scryfall.
You can cover a face-down creature with this reminder card. A face-down creature that was cloaked or cast with disguise has ward {2}. (Whenever that creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)