I've read War of the Spark FAQ and still don't get the point how Bolas's Citadel works with the card that doesn't have a mana cost. One example is Ancestral Vision.

Citadel says that you may play the top card of the library, and if you cast a spell this way, then apply an alternative cost for it. That I understand is Citadel itself doesn't let player to cast a card with restriction, and FAQ says so. But there is an argue about the way Citadel works with a card with no mana cost. So here's the question;

Can I cast Ancestral Vision(or any card with no mana cost printed on paper) with Citadel, paying 0 life?

Hi,

Thanks for your question. The Citadel has an alternate cost - it lets you cast a spell by paying life equal to the card's converted mana cost instead of its normal cost. And the converted mana cost of a card with no mana cost (like Ancestral Vision) is zero. If the Vision is the top card of your library, you'll be able to cast it via the Citadel, and since its converted mana cost is zero, you'll pay zero life to cast it.

Nathan Long
Magic Rules NetRep