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There is a new card called Bolas's Citadel which says:

You may play the top card of your library. If you cast a spell this way, pay life equal to its converted mana cost rather than pay its mana cost.

Can use my life to pay for any X spell?

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    Huh. I'm actually not sure. The relevant rule is 107.3b "If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0." The question is, does "Pay life equal to its converted mana cost" count as an "alternative cost that includes X"? I could see it ruled either way, and I couldn't find any other cards that worked similarly. – Arcanist Lupus Apr 21 at 6:52
  • @ArcanistLupus How does the alternative cost from Bolas's Citadel include X? It literally defines the life cost as equal to CMC, not X, and X is zero outside the stack. – Hackworth Apr 24 at 16:49
  • @Hackworth You can see from my answer that I agree with your conclusion, but I think the part that is tripping people up is that that rule is talking about the situation when you are casting a spell and at that time, the card is on the stack, and in general the calculation of CMC of a spell on the stack does include the chosen value of X. – murgatroid99 Apr 25 at 0:40
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No, you cannot choose any value for X other than 0 when casting cards using Bolas's Citadel.

The phrase "rather than pay its mana cost" or "rather than pay [card name]'s mana cost" is a standard phrase in Magic indicating that the cost specified is an alternative cost. This is stated explicitly in rule 117.9:

Some spells have alternative costs. An alternative cost is a cost listed in a spell's text, or applied to it from another effect, that its controller may pay rather than paying the spell's mana cost. Alternative costs are usually phrased, "You may [action] rather than pay [this object's] mana cost," or "You may cast [this object] without paying its mana cost." Note that some alternative costs are listed in keywords; see rule 702.

So if you cast spells using Bolas's Citadel's second ability, you are required to pay an alternative cost of life equal to the spell's converted mana cost. Then rule 107.3b applies:

If a player is casting a spell that has an {X} in its mana cost, the value of X isn't defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn't apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, "Casting Spells."

So, you are required to choose 0 for X when casting spells with X in the mana cost.

This is confirmed by an official ruling on Bolas's Citadel's Gatherer page:

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

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    You may like to cite rulings on existing cards until we have the WAR rulings article, e.g. the Expertise cycle all include the ruling "If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost." – doppelgreener Apr 21 at 7:35
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    The rules here seem to contain all of the relevant information. I don't see what an existing ruling would add. – murgatroid99 Apr 21 at 16:14
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    Ok, no problem. 👍 I think once the WAR rulings arrive it will at least state it plainly and unquestionably that yes those rules apply like this to this card, but another card's rulings aren't relevant like that. – doppelgreener Apr 21 at 17:44
  • "...nor an alternative cost that includes X" seems relevant here. Wouldn't the life paid include x and therefore satisfy the condition to allow x to be non-0? – km6zla Apr 23 at 15:32
  • I believe that phrase is very literal. The cost specified on Bolas's Citadel does not contain the word "X", so it doesn't satisfy that condition. For an example that goes the other way, Bonfire of the Damned has a Miracle alternative cost that explicitly includes {X}. – murgatroid99 Apr 23 at 17:03

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