, Sacrifice ten nonland permanents: Each opponent loses 10 life.
in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.Bolas’s Citadel turns your library into a supply of spells and your life total into the payment for them. You may look at the top card at any time, play lands from there, and cast spells from there by paying life equal to their mana value instead of their mana cost. Low curves and ways to manage the top card make that ability much more productive.
Sensei’s Divining Top is a natural partner. Its selection ability arranges the next few cards, and its draw ability can clear a card before putting Top itself on the library. You can then cast Top from there for one life. With Aetherflux Reservoir in play, repeating that sequence adds spell casts and life-gain triggers while drawing through the deck. The library and your available life still limit how far you can safely continue.
Citadel’s permissions do not remove normal timing restrictions or grant extra land plays. A land on top can halt the sequence after you have used your land play, making fetch lands, draw effects, and top-deck manipulation valuable. You also need enough life to pay for a spell before any resulting life gain occurs.
Its activated ability offers a separate payoff: tap Citadel and sacrifice ten nonland permanents to make each opponent lose ten life. Token-heavy boards can supply that cost, and Citadel itself may be one of the sacrificed permanents. The card fits black artifact, lifegain, and spell-heavy decks that can support its triple-black cost and turn a powerful engine into a finish before opponents dismantle it.
