On a recent edh game a player was returning all the dragons from his graveyard to play. One of those dragons was Terror of the Peaks, so other dragons would trigger him. All of them enter at the same time, but the active player chooses to have Terror of the Peaks enter first. Is it possible to instant kill Terror of the Peaks and avoid the damage? Some other players were arguing, that the damage is already on the stack, and it does not matter if the dragon is killed. I do not think so.
but the active player chooses to have Terror of the Peaks enter first
That's not possible, if an effect returns all dragons from a graveyard to the battlefield, they all enter at the same time. But Terror of the Peaks' ability still triggers for each other dragon; see for example Cards entering the battlefield simultaneously and "Enter the Battlefield" effects.
Is it possible to instant kill Terror of the Peaks and avoid the damage?
No. It does not matter that the source of the damage is no longer around by the time it resolves. Mogg Fanatic would be a rather stupid card if that was the case. See Does an ability resolve if the source of the ability leaves the battlefield? for more information.
Removing Terror of the Peaks isn't going to do what you hope, but the good news is that there are instant-speed effects that could reduce or eliminate some of the damage. Let's start by looking at the exact wording of Terror's ability:
Whenever another creature enters the battlefield under your control, Terror of the Peaks deals damage equal to that creature's power to any target.
I've highlighted the phrase we're going to take advantage of here.
First, let's note what's not going to work:
- If all the dragons come into play at the same time (via Haunting Voyage or Patriarch's Bidding, for example), then no, all the triggers are going to go on the stack no matter what you do; you can't remove Terror "before" that happens.
- If, while the effects are on the stack, you kill Terror, that's not going to stop the effects from resolving.
- If, while the effects are on the stack, you destroy or exile another dragon, then the effect will still resolve, and it will use the "last known information" rule to still deal the damage
So, what will work? (Some of these are mentioned in comments)
- Countering the resurrection spell
- Exiling Terror from the graveyard while the resurrection spell is on the stack, so it never comes into play
- Reducing the power of one or more of the other dragons, via something like Flunk or Olivia's Midnight Ambush
In this last case, the damage is determined on resolution. Note that if your effect kills the dragon, then the "last known information" rule will see the negative power it had as it died, not the positive power it had originally.