Mausoleum Secrets is a rare Instant card from Guilds of Ravnica Promos (PGRN), collector number 75s. Current market price: — (foil $2.36), TCGplayer market prices via Scryfall.
Undergrowth — Search your library for a black card with mana value less than or equal to the number of creature cards in your graveyard, reveal it, put it into your hand, then shuffle.