Swerve was easily the card I was looking forward to most in Shards. But I have a question about the card itself. What exactly can you play Swerve on? For instance, if your opponent plays an Unmake can you play Swerve and change the target to anything on the field? By that I mean can you take the Unmake and change the target to, let's say, a land and then nullify the spell? For that matter can you Swerve a Counterspell?
–Zak, Lawrence, KS, USA
A: From the Magic Rules Corner:
Swerve's text is clean, simple, and in some ways quite confusing. Let's look at what spells it can target, what spells it can't, and what new targets you are and aren't allowed to choose.
Swerve can target any spell that has a single target, but it can't target any other spells. That means that Unmake, Cancel, Cremate, Blightning, Cruel Edict, and Pacifism, among many other spells in Magic, are all legal targets while they're on the stack. Unmake targets a single creature, Cancel targets a single spell, Cremate targets a single card in a graveyard, Blightning targets a single player, Cruel Edict targets a single opponent (not the same as player), and Pacifism, when it's on the stack, targets the creature it's going to enchant. (After it's in play, Pacifism doesn't target anything.)
If a spell targets "up to" some number of targets, it's a legal target for Swerve only if the number of targets chosen is one.
If more than one target was chosen when a spell was played, or if no targets were chosen, Swerve can't target the spell. That means that Swerve can't target Violent Ultimatum, Agony Warp, Wrath of God, or Mind Spring, among many other spells. It doesn't matter how many legal targets the spell currently has. If you're not sure whether a spell targets, look for the word "target" or the word "enchant." If it doesn't have either of those words, it doesn't have any targets.
If the spell has multiple targets and all but one of them have become illegal, you still can't target the spell with Swerve. For instance, suppose your opponent played Violent Ultimatum targeting two creatures and a land, and you played Tortoise Formation, giving both creatures shroud. Both creatures are illegal targets and won't be destroyed, but Violent Ultimatum still isn't "a spell with a single target" and thus can't be targeted by Swerve.
If a spell targets one thing multiple times, you still can't target it with Swerve. A player who plays Agony Warp, for example, can target two different creatures with the -3/-0 and -0/-3 effects, or he or she can target the same creature with both. Either way, multiple targets were chosen, so Agony Warp is never a legal target for Swerve.
Getting to your second question, let's look at what actually happens when Swerve resolves. You can't change the spell's target to an illegal target, but you have to change the spell's target if you can. You can still play Swerve even if the spell doesn't have any other legal targets, if you want to for some reason.
So you can't change the target of Unmake to a land (or a creature with shroud, or one with protection from white or black, or one that's an illegal target for any other reason). You can, however, change the target of Terror to an indestructible creature such as Spearbreaker Behemoth—it's still a legal target, even though it won't actually be destroyed.
In the same vein, you can change the target of Blightning to the player who played it, but you can't do likewise with Cruel Edict. Cruel Edict must target one of its controller's opponents. In a multiplayer game, you could change Cruel Edict's target to a different opponent of the player who played it, but Cruel Edict's controller is never a legal target for it. The controller of the spell doesn't change when you change its targets, so even if you change the target of a spell like Cremate, the player who played Cremate will still draw a card if Cremate resolves.
One last weird example: Suppose you play a spell, and your opponent plays Cancel targeting it. Those are the only two spells on the stack. You then play Swerve targeting the Cancel. You have to change the target of Cancel if you can, but you can't change it so that Cancel targets itself—a spell on the stack is never its own legal target. You can, however, change Cancel's target so that it targets Swerve. Swerve then finishes resolving and leaves the stack. When Cancel tries to resolve, its target is no longer legal, so it's countered on resolution.
Note also that Swerve itself doesn't target the spell's new target, so it could change Giant Growth's target so that it targets a creature with protection from red, such as Paladin en-Vec.
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