Ok relevant text first

spinta

E Your opponent adds one non-character card from your staging area to your momentum: Add 2 non-character cards from their staging area to their momentum.

Dark mesiah

R commit, discard 1 momentum: Before your opponent adds any number of cards to their momentum, cancel that addition.

What is the reaction here? (if any)

I don't believe there is any interaction in this case. Dark Mesiah specifically refers to a player adding to his own momentum not his or her opponents.When Ira-Spinta's enhance is played by you, it's your opponent who is adding a card to your momentum and not to his or her own.

Zero Cross said:

It shouldn't matter that the momentum gain, which is from your card effect, has the target selected by your opponent. in the end it is your card effect that is adding, reguardless of who chooses.

same reason that blinding rage could dig out multiple copies of your foundations off of your opponents whereabouts unknown, because it was your opponents card that destroyed your card, you got 3 momentum, then you got to search for copies of the foundation that you destroyed.

dark messiah is triggering off of the opponent's card {ira spinta} gaining momentum, it doesn't care if that random guy across the room is asked to choose the foundaton, just that the opponent is gaining momentum off of their card effect.

Back in Block 2, Chester (the character) and Mortal Strike worked together, aka Chester Mill. Although Mortal Strike prevented both players from adding to their own mometum during the combat phase, Chester's ability could still force your opponent to add cards from the top of their deck to their momentum. This is because it is Chester's player who is adding the momentum and not the other player.

I believe that this is the same kind of interaction here. With Blinding Rage it was irrelavent who did the destroying as long as it was due to an opponent's effect. However in this case, it does matter who adds the momentum. If you play Ira-Spinta's enhance you physically place a card to your momentum but it's your opponent who decides which card goes there. If Mortal Strike were legal for this block it would not be able to stop Ira-Spinta.

Zero Cross said:

I agree with this. It is the same thing as Miserable Existence being unable to stop Spinta.

i.e. the opponent (player play spinta) is not adding cards to their momentum, you are adding cards to their momentum.

It's really stupid, and Spinta should have been worded:

E your opponent chooses a non-character card in your staging area that you add to your momentum: you choose 2 non-character cards in your opponent's staging area that they add to their momentum.

It isn't worded that way, and Dark Mesiah does not stop momentum being added by you to your opponent's momentum...

ps. see my hate for card's like Kunai, getting momentum should not be a cost!!!

- dut

If Miserable Existance can't stop this, then Dark Messiah definitely cannot.

Yeah, it can't, as mentioned before, because it's not the player adding cards to their momentum, it's their opponent doing it for them.

Same reason why Mortal Strike can't stop Spinta.