Sunken Citadel is a rare Land — Cave card from The Lost Caverns of Ixalan (LCI), collector number 392. Current market price: $0.59 (foil $0.69), TCGplayer market prices via Scryfall.
This land enters tapped. As it enters, choose a color. {T}: Add one mana of the chosen color. {T}: Add two mana of the chosen color. Spend this mana only to activate abilities of land sources.