Bolas's Citadel is a rare Legendary Artifact card from Magic Online Promos (PRM), collector number 72317.
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.