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I want to use Omnispell Adept's ability

2U, Tap: You may cast an instant or sorcery card from your hand without paying its mana cost.

to cast Entrancing Melody. Its effect reads

Gain control of target creature with converted mana cost X.

What is X for the effect on that card?

3 Answers 3

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The value of X is chosen when the spell is cast. But the only legal choice for X is zero in this case.

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. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

This is confirmed by a ruling on Omnispell Adept.

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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  • As always, there's an exception to this rule: Spoils of War. Its value of X is set by its rules text and thus will not be 0 if cast without paying its mana cost.
    – eyeballfrog
    Commented Jun 21, 2019 at 22:23
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    @eyeballfrog, That is not an exception. 107.3b explicitly doesn't apply to cards whose text define X ("If [...] the value of X isn’t defined by the text of that spell [...]"). Those are covered by 107.3c ("If a spell or activated ability has an {X}, [-X], or X in its cost and/or its text, and the value of X is defined by the text of that spell or ability, , then that’s the value of X while that spell or ability is on the stack. The controller of that spell or ability doesn’t get to choose the value. Note that the value of X may change while that spell or ability is on the stack.").
    – ikegami
    Commented Jun 21, 2019 at 22:59
  • It's not an exception to the explicit rules, but it is an exception to the rule of thumb "X is always 0" that's in the ruling. Perhaps I could have phrased that better.
    – eyeballfrog
    Commented Jun 21, 2019 at 23:05
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    @eyeballfrog, That's a stupid (incorrect) "rule of thumb", and I never said X is always 0. I said "the only legal choice for X is zero in this case". That clearly indicates there are other istuations in which the outcome would be different. The ruling may be a bit imprecise, but I am not at liberty to edit that. That's why I always include the rules on which rulings are based, as well as my own "td;dr" summary.
    – ikegami
    Commented Jun 21, 2019 at 23:55
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Unfortunately for you, X=0. The Comprehensive Rules state:

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. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

This is also mentioned in the Rulings under the card in Gatherer:

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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  • @Arthur and ikegami, thanks, I've corrected my mistake.
    – Glorfindel ♦
    Commented Dec 8, 2018 at 8:56
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I just found the rules clarification on Omnispell Adapts:

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.

In that case, Omnispell Adapt can only cast Entrancing Melody for X=0, which makes the effect on both cards quite useless.

Except on targets like Endless One, because its cost is X=0, when targeted by Entrancing Melody...

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    Token creatures also have a CMC of 0 (unless they're copies of a creature, in which case they have the cloned creature's CMC) so you can steal someone's Wurm token.
    – Veskah
    Commented Dec 8, 2018 at 0:18
  • @Veskah Tokens have a CMC of whatever is defined by the ability that created the token; and if the ability doesn’t define it; then CMC is used. So it’s not automatically 0.
    – GendoIkari
    Commented Dec 8, 2018 at 16:14
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    To be fair the only kinds of tokens that have CMC at the moment are tokens that are copies of things (including Embalm/Eternalize).
    – doppelgreener
    Commented Dec 9, 2018 at 10:47
  • And embalm tokens are specifically given a cost of 0. Seems unlikely that they would ever print an ability that creates a token with a different specific assigned CMC; but they could.
    – GendoIkari
    Commented Dec 9, 2018 at 16:47
  • There are 0 CMC creatures. Orinthopter, for instance.
    – Acccumulation
    Commented Nov 19, 2020 at 5:05

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