Spelltwine

[Mentonin]

If it is not on the stack, where is it?

[Coffee Vampire]

If it is not on the stack, where is it?

Sorry mentonin, I thought there was something off in my answer. Since you cannot normally cast spells targeting other spells while they are in the middle of resolving, I assumed that you couldn't in this case. However, this is not the case. I will explain why you are correct (you CAN target the spelltwine with a counterspell):

The reason you can't target spells while they are in the middle of resolving is that you can only cast spells while you have priority. However, even though you can't interact with spells while they are resolving, they stay on the stack. The last part of the resolution of a spell is it being sent to the graveyard. Now, the reason you are right and I am wrong is that spelltwine ignores normal timing restrictions, allowing you to target spelltwine with a counterspell WHILE it is in the middle of resolving. The reason you can do that is all a counterspell needs is a legal target on the stack. Spelltwine is on the stack so you could target it.

Now if you have {Dispel} as your target, you could not target spelltwine with it, since dispel can only target instants. In that case, you would have to cast the angel's mercy first. Then dispel becomes castable, so you are forced to counter it.

...hope all this crap makes sense. Basically you are not forced to couter angel's mercy if it is negate, because you can target spelltwine. But if it is dispel like in the example, you CAN'T target spelltwine, because spelltwine is a sorcery. So you must counter mercy.

[The1337Magician]

RESOLVED (finally)

[Double-O-Scotch]

I hate to unresolved stuff, but if it was negate, how could you target {spelltwine}? If spelltwine is countered, then the effect of exiling cards from yours and your opponents graveyard would not resolve which is where you got the negate in the first place. It's a paradox. You can't counter spelltwine with a counter grabbed by spelltwine.

[Keyeto]

I hate to unresolved stuff, but if it was negate, how could you target {spelltwine}? If spelltwine is countered, then the effect of exiling cards from yours and your opponents graveyard would not resolve which is where you got the negate in the first place. It's a paradox. You can't counter spelltwine with a counter grabbed by spelltwine.

You can't counter it, but you can target it. You exile the {Negate} and are forced to cast it if able. At this point, it has a legal target. {Spelltwine} has not fully resolved, and is still on the stack, so the {Negate} can target it. Spelltwine finishes resolving, then the remaining two spells are left on the stack. Negate is countered when it goes to resolve, since its only target (Spelltwine) is no longer legal.

[The1337Magician]

So he could've grabbed  {Counterspell} which was actually in his graveyard at the time to "counter" {Spelltwine} and then he would be able to cast {Angel's Mercy}?

[Keyeto]

So he could've grabbed  {Counterspell} which was actually in his graveyard at the time to "counter" {Spelltwine} and then he would be able to cast {Angel's Mercy}?

Indeed. It's a really weird scenario, but it would work out.

[Double-O-Scotch]

I still think it will make the universe implode.