"This Spell Can't Be Countered"
Judge Level 1Counterspell on Abrupt Decay: is that even a legal target — and what happens then?
Cards involved
Abrupt Decay on the Top
Your opponent controls Sensei's Divining Top. You cast Abrupt Decay targeting it — the card says: "This spell can't be countered."
601.2To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell includes proposal of the spell (rules 601.2a–d) and determination and payment of costs (rules 601.2f–h). To cast a spell, a player follows the steps listed below, in order. A player must be legally allowed to cast the spell to begin this process (see rule 601.3). If a player is unable to comply with the requirements of a step listed below while performing that step, the casting of the spell is illegal; the game returns to the moment before the casting of that spell was proposed (see rule 733, “Handling Illegal Actions”).
What happens?
Your opponent holds Counterspell.
Can they target Abrupt Decay with it?
Pick an answer to continue.
"Can't be countered" doesn't make the spell untargetable — it only means the countering part does nothing on resolution. Choosing targets (while casting) and applying the effect (while resolving) are two separate checks.
601.2cThe player announces their choice of an appropriate object or player for each target the spell requires. A spell may require some targets only if an alternative or additional cost (such as a kicker cost) or a particular mode was chosen for it; otherwise, the spell is cast as though it did not require those targets. Similarly, a spell may require alternative targets only if an alternative or additional cost was chosen for it. If the spell has a variable number of targets, the player announces how many targets they will choose before they announce those targets. In some cases, the number of targets will be defined by the spell’s text. Once the number of targets the spell has is determined, that number doesn’t change, even if the information used to determine the number of targets does. The same target can’t be chosen multiple times for any one instance of the word “target” on the spell. However, if the spell uses the word “target” in multiple places, the same object or player can be chosen once for each instance of the word “target” (as long as it fits the targeting criteria). If any effects say that an object or player must be chosen as a target, the player chooses targets so that they obey the maximum possible number of such effects without violating any rules or effects that say that an object or player can’t be chosen as a target. The chosen objects and/or players each become a target of that spell. (Any abilities that trigger when those objects and/or players become the target of a spell trigger at this point; they’ll wait to be put on the stack until the spell has finished being cast.)
They counter anyway
Counterspell goes on the stack, targeting Abrupt Decay. That is legal — even though it won't accomplish anything.
Counterspell resolves — and nothing happens
Counterspell tries to counter Abrupt Decay. That part simply isn't carried out. Counterspell goes to the graveyard, Abrupt Decay stays on the stack.
608.1Each time all players pass in succession, the spell or ability on top of the stack resolves. (See rule 609, “Effects.”)
701.6aTo counter a spell or ability means to cancel it, removing it from the stack. It doesn’t resolve and none of its effects occur. A countered spell is put into its owner’s graveyard.
Decay cleans up
Abrupt Decay resolves and destroys the Top.
Stack
Abrupt Decay
Can't be countered; destroys the target
Counterspell
Target: Abrupt Decay
Sensei's Divining Top
Your opponent's artifact
The stack is empty.
Battlefield
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