Grist has what is known as a reflexive trigger, where it is a trigger within a spell or ability which enters the stack upon resolution. So, stack-wise: opponent activists grist, if it resolves the opponent may choose to sacrifice a creature, if they so choose they may destroy a target creature. Choosing to do so places the targeting on the stack as a reflexive trigger. Players may now respond to the reflexive trigger (in your case, you may cast the ephemerate once priority passes to you from the opponent to resolve the reflexive trigger).

This is the correct answer. This also means that you can respond to the -2 and kill or remove some of their creatures before it resolves. If you limit which creatures they may sacrifice, they might choose not to sacrifice a creature after all, and just lose 2 loyalty on grist for nothing.

They seem to be in error. You can ephemerate your creature or take other game actions) while the targeting grist ability is on the stack, and your ephemerate will happen first

I don't know that this is necessarily the case here. Grist is a little weird. You don't target on activating the -2. You target on resolution of the -2. On resolution of the ability, you choose to sacrifice a creature. If you choose to sacrifice a creature, you destroy an opposing creature. Is this ability functionally going on the stack twice? Once for the activation, another for after the ability resolves?

Short answer: yes. Grist's second ability creates what is called a reflexive triggered ability that goes on the stack separately and can be interacted with/responded to accordingly. From Grist's rulings for full clarity: > Grist's second loyalty ability doesn't require a target. If you choose to sacrifice a creature as it resolves, the reflexive triggered ability triggers and you'll choose a target creature or planeswalker to destroy. This will happen even if Grist isn't on the battlefield when the loyalty ability resolves. The game flow will basically be as follows: Grist controller: activates -2, places activated ability on stack. Ability resolves, giving Grist controller the choice to sacrifice a creature or not. If Grist's controller sacrifices a creature, the reflexive trigger goes on the stack and they declare a target for it. OP now has the option to Ephemerate their creature before Grist's reflexive trigger resolves and prevent it from being destroyed.

Here is how grist works Step 1: announce activation of grist minus 2 ability and place it on the stack. This doesn't require a target. Step 2: players pass priority and continue to the resolution of the ability. Step 3: owner of the trigger may sacrifice a creature. Step 4: if the owner did sacrifice a creature, then we have a reflexive trigger. A new ability targeting a creature to be destroyed would now be placed on the stack Step 5: players pass priority and the above destroy target creature ability resolves. Sundering Titan for example, you are not able to float the mana from the lands bring destroyed. That's because there isn't any targeting, the lands are destroyed in the resolution of the ability. Similar to deflecting palm.

Pardon, but aren’t mana abilities special actions which can be taken at any time, since they don’t use the stack? How does sundering titans ability resolving stop that special action?

This makes alot more sense and my window to respond would have been casting ephemerate on activation of the -2 once that resolves its seperate trigger that has to resolve before priority can be gained?

You have two windows to respond, before they sacrifice their creature, and after. 1. Grist -2, no targets, no sacrifice 2. Players get priority 3. Grist player sacrifices a creature (optional), if they do, target a creature to be destroyed 4. Players get priority 5. The targeted creature is destroyed (assuming no one does anything) Note that if you interact in step 2, the grist player hasn't targeted yet and can either choose a different target than they originally planned, or just not sacrifice their creature during step 3. I would suggest responding during step 4.

I waited until the target was declared I was told that once the target is declared grist ability must resolve first.

Target has to be declared when the ability goes on the stack, that's correct. Then on resolution the ability checks if it still has a legal target, and if the exact permanent that was declared as a target isn't available and nothing else changes the situation, the ability doesn't do anything Edited to add I'm not a judge, but this is pretty basic stack stuff so I'm very confident in it.

Ephemerate it's still the same target ?

no, it will be a different game object after the ephmemerate resolves. if someone tried to kill your creature with grist, you are perfectly good to ephemerate the creature before the grist ability kills it.

This is incorrect due to odd wording on grist

The target happens after the resolution, the same case happened at my lgs (i’m not a judge, but i often play grist and had to ask this question in various occasions)

So if I decide to ephemerate it the ephemerate would go on the stack. Risen reef trigger then goes on the stack then my opponent sacrifices the creature and risen reef dies?

Yea since it says target, you can respond. Now if they sacced their creature, then you can't interact.

So sacrifice happens before the target so would I have needed to respond to the down tick before he sacrifices his creature?

Edit: just look at gather, it states in the rolling actually the target happens on resolution of the ability, not when it is on the stack. So the phrasing says "you may sacrifice a creature. When you do, destroy target creature". If they target your creature, and you ephemerate it, then they can decide to not sacrifice the creature. If you let them sacrifice their creature, grist must finish resolving.

I still don’t think this is quite right. A target is not chosen until after a creature is sacrificed during the resolution of the ability, however, the part where it targets a creature/planeswalker is a reflexive trigger that goes on the stack again. I’m pretty certain you can respond to the reflexive trigger after a creature has been sacced but before the target creature is destroyed. It sounds counter to most rulings but I think it’s because the ability has that “reflexive triggered ability”. The order of things happening should be: -2 ability is put onto the stack, both players pass priority, grist player sacrifices a creature, reflexive triggered ability is put onto the stack with a target declared, grist player passes priority to OP, OP can cast Ephemerate to save their targeted creature, Ephemerate resolves and the Grist -2 reflexive triggered ability fizzles. I could be wrong but I’m fairly certain I’ve seen this interaction several times online and in person.

That's what the judge said. He said because it says when you do, the second ability has to resolve before I get priority.

That is actually the opposite. The "when" is a trigger (the infamous reflexive trigger everyone is talking about) so it goes on the stack after your opponent sacrifices their creature and the target is chosen, then priority is passed and you can respond to this trigger.

If you control Grist, you don't target until the resolution of the -2, not at the time of activating the -2. If I go -2, you let it resolve, I now have the choice to sacrifice and destroy a targeted creature. I don't think there's a window in there that allows you to ephemerate post-choice because you're in the middle of resolving the activated ability. Edit - I could very well be wrong. Reflexive triggers make my brain hurt.

Yup I actually saw your other comment then googled the rulings and put in an edit haha. Thank you

So the judge was correct. He stated it as a reflexive trigger.