Sunken Palace is a rare Land — Cave card from Modern Horizons 3 Commander (M3C), collector number 81. Current market price: $1.15, TCGplayer market prices via Scryfall.
This land enters tapped. {T}: Add {U}. {1}{U}, {T}, Exile seven cards from your graveyard: Add {U}. When you spend this mana to cast a spell or activate an ability, copy that spell or ability. You may choose new targets for the copy. (Mana abilities can't be copied.)