It depends how you stack it, there are many.......Possibilities! Sorry.
Neera's ability needs the spell to still be on the stack, so if it is removed by Krark/Storm first, you cannot get the 'cascade' trigger because you do not return the spell to the bottom. However, Krark/Storm both go off regardless of whether or not the spell is still there. The important phrase on Neera is "If you do," which requires you to actually perform the listed action.
If you stack Neera's trigger first, and then Krark on top, it resolves in reverse order. So you flip, if you win, you cast & resolve your copy, then resolve your choice on Neera's trigger. If you lose the flip, the spell is returned to your hand, and Neera's trigger doesn't do anything, or rather can't do anything.
If you stack the other way, you can resolve Neera's trigger first and bottom the spell, cast your other thing, and then resolve Krark's trigger, which at that point is just upside - if you win you still get a copy of the bottomed spell, but if you lose the spell is already bottomed so you don't return it to hand. Also regardless of how you stack, if Neera hits another instant/sorcery, that will trigger Krark as well and resolve normally, so the new spell can wind up back in your hand.
With Possibility Storm, its basically the same thing, but neither will re-trigger the other, because of once per turn/cast from hand. So if you place the Storm trigger first, then Neera on top and choose bottom the spell, you can get both effects.
If an opponent controls P.Storm however, it depends whose turn it is, because triggers are placed in APNAP (active player/non-active player) order. On your turn, you cast a spell and trigger your Neera and their Storm, you place your trigger first, then theirs goes on top. Then Storm does its' thing first, and by the time your Neera comes up, the original spell is gone. But if it's your opponent's turn, then the opposite happens, you can get Neera's cascade and the OP's P.Storm. And if its a multi-player game and its neither player's turn, then its still turn order, so if they are before you, you double-trigger, if they are after you, they eat your Neera trigger.
If Neera's trigger resolves and you don't have the spell on the stack any more (like losing the Krark trigger coin flip if you resolved that trigger first, or the spell gets countered, etc.) or choose not to put the card back, you don't get the rest of the trigger's effect due to the "if you do" clause. If you win the Krark coin flip, you get a copy of the spell even if that spell is gone already (like if the Neera trigger put it away). If Neera gets you another instant or sorcery spell and you cast it, Krark will trigger again.
2) Neera + PS
You either won't get anything from Neera if you exile the spell with PS first. Or you use Neera's trigger first, get another spell from your library (which won't trigger PS again, since you are not casting it from your hand), then proceed to get another spell from PS after resolving the new spell from Neera. It can't exile the spell anymore, but the rest of the effect is not depending on doing so, since there is no "if you do" clause. And Neera can only trigger once per turn, so it won't trigger off the new spell either.
Figure it's worth coming at this one with several explanatory styles.
Whenever you cast a spell, but only the first time while Neera is in play each turn, a triggered ability for Neera goes off, and, if it is instant or sorcery, Krark's does also.
If you have Krark's trigger ordered on top of Neera's trigger, then it will resolve first, and, if it makes a copy, you resolve the copy, then resolve the Neera triggered ability. If Krark's coin comes up loss, the spell is put into its owner's hand, which will force the Neera triggered ability to do nothing, because its first instruction - you put the spell on the bottom of its owner's library - is impossible, and the rest of the effect depends on you choosing to do that. Notwithstanding this, you may use Neera to substitute your spell for another card; you may also choose to put the spell away and not cast the replacement. If you do cast the spell, and that spell is instant or sorcery, it triggers Krark's thumb. At that point, you just have a Stack consisting of that spell, and a Krark trigger, the resolution of which I leave as an exercise for the reader.
You might also choose to decline the entirety of Neera's ability. Then, if you did indeed *choose* that (without Krark having nay-nay'd the spell because of the coin toss), you will have the spell you cast first in front of you.
If you have Neera's trigger ordered on top of Krark's trigger, then it will resolve first. If you choose to use that ability to tuck the spell under your library, the Krark triggered ability may still eventually flip a winning coin toss, which will create a copy of the spell you had cast in the first place. First, though, you have the choice, if you did tuck the first spell, to cast the second card that gets revealed. Casting that spell, if instant or sorcery, also triggers Krark. Krark may copy or return that revealed card; the next things to do are resolve that copy and that revealed spell, or to do nothing since neither one exists.
After the resolution of Neera's trigger, what happens depends on if you had replaced the spell.
If you did not cast the replacement for the spell, you will next handle the Krark's trigger (first one). It can copy the first spell in this scenario even if you put it under your library anyway, copying what it looked like before it left "the zone it was expected to be in." If you lose its flip, that instruction is ignored because the object is not in "the zone it was expected to be in." Last, if you had indeed won, you will have a copy to resolve, and then possibly the original spell if you did not tuck it; and, if you lost, you will have simply the original spell, unless you put it away before now.
But, if you chose to cast the card that Neera's ability revealed, then, the next thing to happen is not to handle Krark's trigger but to resolve that revealed card, since it was cast while Krark's trigger was on the Stack. That spell will resolve. Afterward, the Krark's trigger may, again, be able to copy the first spell even though it is not here, and, if Krark's toss loses, it can't do anything to the spell that isn't here. If Krark wins, you get a copy which is the only thing on the Stack now; or, if you lose, the Stack is now empty.
Note that I have skipped over the parts where players pass priority in this sequence, assuming that no one chooses to interact with what's going on. If you want to know something more specific, ask about what you want to know.
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Neera's ability needs the spell to still be on the stack, so if it is removed by Krark/Storm first, you cannot get the 'cascade' trigger because you do not return the spell to the bottom. However, Krark/Storm both go off regardless of whether or not the spell is still there. The important phrase on Neera is "If you do," which requires you to actually perform the listed action.
If you stack Neera's trigger first, and then Krark on top, it resolves in reverse order. So you flip, if you win, you cast & resolve your copy, then resolve your choice on Neera's trigger. If you lose the flip, the spell is returned to your hand, and Neera's trigger doesn't do anything, or rather can't do anything.
If you stack the other way, you can resolve Neera's trigger first and bottom the spell, cast your other thing, and then resolve Krark's trigger, which at that point is just upside - if you win you still get a copy of the bottomed spell, but if you lose the spell is already bottomed so you don't return it to hand. Also regardless of how you stack, if Neera hits another instant/sorcery, that will trigger Krark as well and resolve normally, so the new spell can wind up back in your hand.
With Possibility Storm, its basically the same thing, but neither will re-trigger the other, because of once per turn/cast from hand. So if you place the Storm trigger first, then Neera on top and choose bottom the spell, you can get both effects.
If an opponent controls P.Storm however, it depends whose turn it is, because triggers are placed in APNAP (active player/non-active player) order. On your turn, you cast a spell and trigger your Neera and their Storm, you place your trigger first, then theirs goes on top. Then Storm does its' thing first, and by the time your Neera comes up, the original spell is gone. But if it's your opponent's turn, then the opposite happens, you can get Neera's cascade and the OP's P.Storm. And if its a multi-player game and its neither player's turn, then its still turn order, so if they are before you, you double-trigger, if they are after you, they eat your Neera trigger.
Easy, right?
If Neera's trigger resolves and you don't have the spell on the stack any more (like losing the Krark trigger coin flip if you resolved that trigger first, or the spell gets countered, etc.) or choose not to put the card back, you don't get the rest of the trigger's effect due to the "if you do" clause. If you win the Krark coin flip, you get a copy of the spell even if that spell is gone already (like if the Neera trigger put it away). If Neera gets you another instant or sorcery spell and you cast it, Krark will trigger again.
2) Neera + PS
You either won't get anything from Neera if you exile the spell with PS first. Or you use Neera's trigger first, get another spell from your library (which won't trigger PS again, since you are not casting it from your hand), then proceed to get another spell from PS after resolving the new spell from Neera. It can't exile the spell anymore, but the rest of the effect is not depending on doing so, since there is no "if you do" clause. And Neera can only trigger once per turn, so it won't trigger off the new spell either.
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Whenever you cast a spell, but only the first time while Neera is in play each turn, a triggered ability for Neera goes off, and, if it is instant or sorcery, Krark's does also.
If you have Krark's trigger ordered on top of Neera's trigger, then it will resolve first, and, if it makes a copy, you resolve the copy, then resolve the Neera triggered ability. If Krark's coin comes up loss, the spell is put into its owner's hand, which will force the Neera triggered ability to do nothing, because its first instruction - you put the spell on the bottom of its owner's library - is impossible, and the rest of the effect depends on you choosing to do that. Notwithstanding this, you may use Neera to substitute your spell for another card; you may also choose to put the spell away and not cast the replacement. If you do cast the spell, and that spell is instant or sorcery, it triggers Krark's thumb. At that point, you just have a Stack consisting of that spell, and a Krark trigger, the resolution of which I leave as an exercise for the reader.
You might also choose to decline the entirety of Neera's ability. Then, if you did indeed *choose* that (without Krark having nay-nay'd the spell because of the coin toss), you will have the spell you cast first in front of you.
If you have Neera's trigger ordered on top of Krark's trigger, then it will resolve first. If you choose to use that ability to tuck the spell under your library, the Krark triggered ability may still eventually flip a winning coin toss, which will create a copy of the spell you had cast in the first place. First, though, you have the choice, if you did tuck the first spell, to cast the second card that gets revealed. Casting that spell, if instant or sorcery, also triggers Krark. Krark may copy or return that revealed card; the next things to do are resolve that copy and that revealed spell, or to do nothing since neither one exists.
After the resolution of Neera's trigger, what happens depends on if you had replaced the spell.
Note that I have skipped over the parts where players pass priority in this sequence, assuming that no one chooses to interact with what's going on. If you want to know something more specific, ask about what you want to know.
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