Bolas's Citadel is a rare Legendary Artifact card from War of the Spark Promos (PWAR), collector number 79s. Current market price: — (foil $37.66), TCGplayer market prices via Scryfall.
You may look at the top card of your library any time. You may play lands and cast spells from the top of your library. If you cast a spell this way, pay life equal to its mana value rather than pay its mana cost. {T}, Sacrifice ten nonland permanents: Each opponent loses 10 life.