Hey folks,
In the most recent Arcane Duels League, we saw a new trick used to counter a Ballista's attack. Blur was revealed after the Ballista declared its target, and the players agreed that this canceled the attack. Here is the game with time stamp:
https://youtu.be/ywZ2lpGRHIQ?t=287Initially my gut told me that this did not work, but could not recall where I had read the clarification and could not pinpoint the rule. It is, as the judges conclude, legal to reveal Blur when the attack has begun. So why should it not work?
Today I had an epiphany: The reason why the trick does not work is because you can only reveal enchantments "after a step or phase". The first thing that happens when the Ballista is about to fire is the turning of the ready marker, this is not a step. The first "step" taken in a Ballista's ranged attack is "Declare Attack". In the Declare Attack Step you declare the target and check for Line of Sight, this is the only point at which LoS is checked in an attack. After the Declare Attack Step, you may reveal Blur, but it is too late.
It is written in the base manual under "Attacks Step 1: Declare Attack", although they do not put a whole lot of weight on the ruling here. I have scoured the forum to find a clarification of this rule and I have to quote myself on this, from my Enchantment Tranfusion Clarification:
[mwcard=MWSTX1CKE04]Enchantment Transfusion[/mwcard] (ET)
Spells that interact with Line of Sight (Lesser Invisibility/Blur):
If your opponent targets a creature with a spell, you may reveal ET after the opponents ”Pay Cost” step to move an enchantment that would cause LoS to be broken, and the spell is cancelled.
Note that this does not work against standard non-spell attacks, since LoS is only checked during the first step "Declare Attack" and you cannot interrupt an initiated step. You would have to move the LoS-breaking enchant before the attack is even declared.
No one has ever told me that this was wrong, so I assume it is still true.
Zuberi has a similar post for spells, that can be used to support the ruling:
Addendum regarding Option 2: Mana Siphon doesn't choose a target mage until the Resolve Spell Step (the spell itself is targeting a zone, it is the effect of the spell that targets a mage), which is why the Divine Intervention trick works. Similarly, sweeping doesn't choose it's secondary target until after it finishes attacking the first target (luckily the second target is now optional). In order to force someone to choose a different target, possibly making them target themselves, you have to make their desired target illegal before they've actually had a chance to officially target it. For most spells, this means you must make the thing an illegal target before Step 1: Cast Spell, which will probably result in them not casting the spell at all rather than casting it on an undesirable target. Once they've officially selected a target, it is impossible to change it, much less force them to change it.
In conclusion, Blur cannot be revealed to break a Ballista shot or any other attack, you have to reveal it before the attack is declared.
If you want to write it in a more logical, less rules lawyery way: You can reveal Blur after the attack is declared, but the Ballista bolt is already fired and on intercept course with the creature, it does not care that your creature turns blurry/invisible at this stage.
Feel free to discuss and tell me if I am wrong.