The X Factor
Secrets of Strixhaven just released this weekend, and one of the five colleges featured in the set, Quandrix, has a lot of cards with X in their cost or cards that care about spells with X in their cost, and consequently I got a number of judge calls about them during prerelease. So here's a comprehensive post about how X spells work, starting from the basics.
What does X Mean?
X is a variable. A placeholder. When you cast a spell with an X in its mana cost, you choose what you want the value of X to be. If I have a Blaze in my hand and six Mountains on the battlefield, I can cast Blaze with X equaling 5 (the sixth mountain being used to pay for the red mana symbol next to the X in the mana cost) and deal five damage to something.
Alternatively, I could choose X to be 2 and cast Blaze using three of my Mountains, leaving my other three lands to cast a three-mana creature that turn.
I could even choose X to be 0, and pay for Blaze with only a single Mountain! Blaze won't deal any damage to anything, but it will still target something (perhaps I want to trigger a repartee or valiant ability).
(And in case you're wondering—no, X can't be a negative value. Nice try, though.)
Some spells will have an effect that uses X, but don't have an X in their mana cost. They might have an additional cost that defines what X is, like in the case of Toxic Deluge:
Toxic Deluge works very similarly to Blaze: you announce what you want the value of X to be as you begin to cast the spell. Instead of paying that much additional mana, you pay that amount of life as you're paying the costs of the spell.
Some cards have activated abilities with X in their activation cost, like with Steel Hellkite:
The steps for activating an ability are essentially the same as casting a spell: you announce that you're activating the ability, declare what the value of X is, choose any targets (if there are any; Steel Hellkite's ability doesn't), then pay costs.
For these spells and abilities where X is defined in the cost, the value of X is "locked in" by the time the spell is cast/ability is activated, and opponents can respond to the spell/ability on the stack knowing what the value of X is.
On the other hand, some effects use X but don't define X with the spell or ability's cost. Take a look at the triggered ability on Craterhoof Behemoth:
Here, the value of X is determined as the ability resolves. If I cast Craterhoof Behemoth as my fifth creature, and then my opponent kills three of my creatures while the enters-the-battlefield ability is on the stack, my creatures will only get +2/+2 until end of turn when the ability resolves.
Some spells have X in their cost but don't use X in their effect. We can see an example of this with the new card Molten Note:
How much damage Molten Note deals is determined by the total amount of mana spent to cast it. If you cast it from your hand and choose X = 2, then you'll be spending four mana to cast it and it'll deal 4 damage.
Some spells and abilities have multiple X's in their costs, and this is what seems to really trip people up. It just means you need to pay X mana that many times.
Let's say you want to return two instants/sorceries to your hand with Divergent Equation. How much mana do you need to spend to cast it? Well, we know X = 2, and there are two X's in the mana cost. Including the blue mana in the cost, you're going to need to pay five mana in total. Let's have another example with Crackle with Power. How much mana do you need to cast a Crackle With Power that can deal 20 damage to your opponent?
First off, note that X is used to determine not only the amount of damage that this spell deals, but also the number of targets it has. If it has multiple targets, you'll need to choose that many different targets; you can't cast it X = 2 and say that you'll choose your opponent to be both targets.
With that in mind, how big does X need to be? Since Crackle deals five times X, and we want to deal 20 damage, X needs to equal 4. If X equals 4, because there are three X's in the mana cost, plus those two red mana symbols, you'll need to spend fourteen mana in total. (If you're looking to kill your opponent in one shot, Crackle With Power is a lot more mana-efficient than Blaze!)
Mana Value
Players frequently have questions in regards to X spells and what their mana value is. (If you're an old-time Magic player, you may know "mana value" as "converted mana cost"—the terms are equivalent).
Here's how you determine what an X spell's mana value is: if the card is in any zone other than the stack, for the purpose of determining mana value, X is 0. If it is on the stack, then the mana value does include the value of X. What does this look like in practice? Well, it means I can use Starfield Shepherd...
...to tutor up an Endless One...
...cast it for X = 10...
...and my opponent can counter it with Disdainful Stroke. Or they can let it resolve and kill it with Requiting Hex.
On the stack, this Endless One's mana value is 10. In the other zones, its mana value is 0.
Copying X Spells
If you copy a spell with X in its cost while it's on the stack, the copy will retain the value of X. So if you cast Endless One with X = 5, hold priority, and cast Double Major...
...you'll create a copy of your Endless One spell, and when the copy resolves, it'll become a token Endless One with five +1/+1 counters on it. Similarly, if you cast Blaze with X = 5, and cast Twincast to copy it...
...the copy of Blaze will also deal 5 damage.
Note that this only applies to copying X spells while they're on the stack. Permanents don't retain the value of X they were cast for as a copiable value. So if there's an Endless One on the battlefield with five +1/+1 counters on it (presumably because it was cast earlier for X = 5), and someone tries to create a token copy of it with Rite of Replication...
...they're probably not going to be happy with the outcome: a 0/0 token copy that dies as soon as state-based actions are checked. (Or, if Rite of Replication was kicked, five 0/0 token copies that die as soon as state-based actions are checked.) You also need to be careful about copying X spells that don't actually use the value of X—I mentioned Molten Note earlier—another example of this sort of X spell is Prismatic Ending:
If you Twincast a Prismatic Ending, while the copy may technically retain the value of X, the effect of the spell depends on the colors of mana spent to cast it, not the value of X. Since the copy wasn't cast, that number is zero. So you could exile a nonland permanent with mana value of 0 with the copy, but that's it.
Free Casting With X
Any time you would cast a spell with X in its cost "without paying its mana cost", X must be 0. You can't pay additional mana into X. I know it's disappointing when you flip an X spell off an Urza activation, but sometimes that's just how the cookie crumbles.
(On the other hand, you still have an Urza, Lord High Artificer on the battlefield. I think you'll land on your feet.)
There is a small exception here: if the spell that you're casting "without paying its mana cost" has an X not in its mana cost but as part of an additional cost, like kicker, then you can spend mana into X. Let's say your Urza activation reveals a Thieving Skydiver:
You can cast it without paying its mana cost, choose to kick it for, say, X = 3, and pay three mana, stealing an opponent's Worn Powerstone when the Thieving Skydiver enters the battlefield.
Corner Cases
Unbound Flourishing does something that, as of yet, no other Magic card does: it alters spells' X values. This can lead to some interesting outcomes.
Let's say you control Unbound Flourishing and tap six lands to cast Hydroid Krasis, X = 4.
When the Hydroid Krasis becomes cast (it's now on the stack, it hasn't resolved yet), both its own "When you cast this spell" ability and Unbound Flourishing's first ability trigger, and you choose the order that those two abilities are placed on the stack; in other words, you get to choose whether you double X before you gain life and draw cards, or after. I imagine that most of the time you'll want to gain more life and draw more cards, but hey, maybe you only have three cards in your library!
This is also the case for permanents that trigger off of other spells with X in their mana cost being cast. If you, in the previous example, had Brass Infiniscope on the battlefield in addition to Unbound Flourishing...
and tapped it to help cast Hydroid Krasis (still cast for X = 4), you would—just like the Hydroid Krasis's cast trigger—get to order the Infiniscope's delayed triggered ability to resolve after the Unbound Flourishing's trigger, and it will also use the modified value of X. Where you would gain 2 life without Unbound Flourishing, you can instead gain 4 life.
I want to touch on some corner cases where a card may have multiple X's that represent different values. Normally, all X's on one card represent the same value; in the rare cases where a card needs to use two different variables, Wizards might add a Y to the mix, like on Phyrexian Ingester.
Sometimes, though, cards will gain text or abilities that have X from other cards, and that can lead to situations where a card has, well, different X's on it.
For a more obvious example, say you have an Endless One with three +1/+1 counters on it as well as an Agatha's Soul Cauldron that has exiled a Steel Hellkite.
The Endless One gains the activated abilities of Steel Hellkite. If you activate the one with X in it, the value of X that the ability uses (as you might expect) is the amount you spent to activate it, not the value of X for which you originally cast Endless One.
Here's a less obvious example: what if you cast a Mockingbird, X = 3, and choose to copy an Endless One as it enters? Will it enter with three +1/+1 counters on it?
Unfortunately, no. Even though Mockingbird was cast with an X value, it's not the same X that the ability it gains uses, and because that ability doesn't itself define what X is equal to, the game defaults to 0. Mockingbird enters as a 0/0 Eldrazi Bird with flying and no counters on it. This is a tricky one! If you'd like to look up the relevant rules citation, it's 107.3j.
Well, that's all the stuff about X spells that I can think to include. Thanks for reading! If you have any questions involving X spells, my ask box is open.











