If an Attacker can cancel dice results (Wampa, Flechette cannon, etc) and a defender can cancel dice results ( Countdown), who cancels what when and how does it resolve? Initiative?

Defender modifies Attack dice first.

Attacker modifies Defense dice first.

All the Cancel Dice effects except Accuracy Corrector happen during Compare Results.

In the unlikely event that it ever matters, I assume the player with initiative would resolve their cancel effect first.

Edit: An event where it matters was pointed out in the OP. *headdesk* But as @StevenO says later, it looks like Wampa has first crack at it.

We used to have a rule that said that effects that cancel dice happen during Compare Results unless otherwise specified, but that rule got left out of the Rules Reference when they put out the new core set. So now we have to ask every time a new effect that cancels results comes out. So far, as far as I know, they still all happen during Compare Results (again with the exception of Accuracy Corrector, which has to happen during Attacker Modifies Attack Dice to have its intended effect).

When it comes down to Wampa vs. Countdown it seems that both can cancel dice during the Compare Results step. It appears that Wampa triggers "At the start of" that step while Countdown just occurs during that step; while initiative may determine a tie it seems to me that Wampa gets the first chance to cancel the attack dice at the start of the step which will trigger his ability. Of course after this Countdown can still trigger but I'd STRONGLY recommend against that because while you can still cancel no dice the damage would still happen in that case.

Of course the Compare Results step happens after defense is done rolling and getting modified so everyone should know what can happen. If Wampa will get the crit through then without any shield to stop it there is little reason for Wampa to cancel the dice in exchange for just one face-down card instead of dealing a face-up card and possibly more if more results would get through defense. If Wampa is going to let the dice go then Countdown probably wants to use her ability and cancel them for a single hit. All said Countdown should only be taking one face-down card under almost any circumstance.

Wampa is locked to 'start of compare results'

Countdown has the option of being anytime during the compare results step, including the start of the compare results step.

If the countdown player wishes to resolve countdown at start and this contests with their opponents Wampa, initiative decides.

Wampa doesn't have the option of delaying his effect. Countdown does, but he doesn't have to.

Or maybe if both trigger at the same time you could use both at the same time. Let Countdown start the cancellation, Wampa finish the cancellation and deal a card and then have Countdown deal a card.

RAW Countdown might not work, because' during' is the part where you actually suffer damage. Traditionally cancellation effects happened at the start of the Compare results check.

Start of phase always come before during a phase. So RAW Wampa happens before Countdown.

14 hours ago, McLaine said:

Wampa is locked to 'start of compare results'

Countdown has the option of being anytime during the compare results step, including the start of the compare results step.

If the countdown player wishes to resolve countdown at start and this contests with their opponents Wampa, initiative decides.

Wampa doesn't have the option of delaying his effect. Countdown does, but he doesn't have to.

I don't think we've got precedent for treating "during" as equivalent to "at the beginning" or "at the end" or "after the beginning but before the end" at the player's option. I think effects that happen at the beginning of a phase are strictly before effects that happen "during" that phase, but I can't think of a good test case off the top of my head.

In any case, unless Countdown happens to get a shield token somehow I'm not sure it should ever matter who is doing the cancelling as Countdown is going to be taking a card anyway it goes.

4 hours ago, StevenO said:

In any case, unless Countdown happens to get a shield token somehow I'm not sure it should ever matter who is doing the cancelling as Countdown is going to be taking a card anyway it goes.

Countdown gives themselves a stress too

29 minutes ago, nigeltastic said:

Countdown gives themselves a stress too

All the more reason to let Wampa cancel dice first "at the start of the Compare Results Step" which I believe is what most of us would advocate is the proper timing.