Fomori Vault is a mythic Land card from The Big Score Promos (PBIG), collector number 29p. Current market price: $13.67 (foil $12.04), TCGplayer market prices via Scryfall.
{T}: Add {C}. {3}, {T}, Discard a card: Look at the top X cards of your library, where X is the number of artifacts you control. Put one of those cards into your hand and the rest on the bottom of your library in a random order.