The Key to the Vault is a rare Legendary Artifact — Equipment card from Outlaws of Thunder Junction (OTJ), collector number 320. Current market price: $1.30 (foil $2.34), TCGplayer market prices via Scryfall.
Whenever equipped creature deals combat damage to a player, look at that many cards from the top of your library. You may exile a nonland card from among them. Put the rest on the bottom of your library in a random order. You may cast the exiled card without paying its mana cost. Equip {2}{U}