Sunken Citadel is a rare Land — Cave card from The Lost Caverns of Ixalan (LCI), collector number 285. Current market price: $0.49 (foil $1.02), 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.