Blu, Mansion Prince is a rare Legendary Creature — Human Noble card from Unknown Event (UNK), collector number RU04d. Current market price: $40.78, TCGplayer market prices via Scryfall.
Whenever this creature enters or attacks, investigate. Whenever you sacrifice a Clue, choose a Room card at random. Create a token that's a copy of one of its halves, then unlock it. (There are currently 30 Room cards.)