Interaction between Nivmagus Elemental and Rune Snag

Question

Recently the following situation popped up in a casual game and it was a bit confusing. I was wondering if someone here could help out.

• Player T has 4 mana open and plays a 2 mana spell in his main phase
• Player G has a Nivmagus Elemental out and plays a Rune Snag to counter T's 2 mana spell; Player G has no Rune Snags in his graveyard
• T decides to pay the 2 mana required to not have his spell countered.
• At this point G exiles his rune snag using Nivmagus Elemental to obtain two +1/+1 counters

It seemed to me that this cannot happen, since either Rune Snag resolves and T pays two mana, or Nivmagus Elemental eats Rune Snag and T does not need to pay 2 mana since it never resolves.

Does player T need to pay two additional mana to play his spell even if Nivmagus Elemental eats it?

Answer 1

You are correct, Player T should not have had to pay 2 mana.

The decision of whether or not to pay 2 mana happens as part of Rune Snag resolving. Player G has 2 options:

1) Let Rune Snag resolve, in which case Player T needs to pay 2 mana (or have their spell countered), but Nivmagus Elemental won't do anything, as Rune Snag will no longer be a valid target for the ability.

2) Activate Nivmagus Elemental's ability while Rune Snag is on the stack, exiling Rune Snag. Player T will not have to pay 2 mana, because Rune Snag will never resolve.

Note that option 1 will happen by default if player G does not specify otherwise. Player G has the right to respond to their own spell:

116.3c If a player has priority when they cast a spell, activate an ability, or take a special action, that player receives priority afterward.

However, they must specifically state that they wish to retain priority:

(From the tournament rules):

Whenever a player adds an object to the stack, they are assumed to be passing priority unless they explicitly announce that they intend to retain it.

So if player G casts Rune Snag without saying anything, player T can simply say "ok" or "resolves" or "I'll pay the mana", and it will be too late for player G to use Ninmagus.

This rule matters especially for Ninmagus in general; the player will always need to declare that they wish to retain priority if they want to ensure that they can use it.

Answer 2

You are correct that this situation cannot play out as described.

Any time that a spell or ability asks a player to pay mana as part of its effects, that player has an opportunity to activate mana abilities while the spell is resolving, immediately before making the payment. So, in this situation, if player T taps lands and says something like "I pay for Rune Snag", the default assumption is that Rune Snag is resolving. At that point, it is too late for player G to activate Nivmagus Elemental's abilities.

There is a partial exception to this. If player G says "In response to Rune Snag" when activating the mana abilities, or casts a spell like Dark Ritual, or activates a mana ability that can only be activated at instant speed, like on Lion's Eye Diamond, then they are taking those actions before Rune Snag resolves, so that gives player G an opportunity to activate Nivmagus Elemental's ability and exile the Rune Snag. If player G does that, player T does not need to pay any mana for Rune Snag.

Answer 3

When any spell or ability, or in this case the cost of an ability, interacts with or targets a spell, it can only interact with or target a spell on the stack. Cards, or non card objects like tokens, aren't considered spells anywhere else by the game. This means that once the spell has resolved (causing player G to pay 2 mana or let the snag counter their spell) it is off the stack and can't be interacted with or targeted by anything that targets spells.

This is the same way you can't Remove Soul/Essence Scatter a creature that has already entered the battlefield, it's no longer on the stack, so it's no longer a spell. Cards like Mystical Tutor which search the library search for an instant or sorcery card, because the card is not yet a spell.

In your specific case Nivmagus Elemental interacts with an instant or sorcery spell you control by exiling it to activate its ability. If you allow the spell to resolve, and allow the opponent to pay for the Rune Snag the spell has left the stack and is no longer a spell you control for Nivmagus to exile, it's already in the graveyard.