The power will deal damage to anyone hit before you are hit!
I think it is helpful to clarify order-of-execution in scenarios like this.
- Declare target(s)
- Make Attack Roll(s)
- Make Damage Roll
My original thought was that you do all the declaring, then roll all the attacks, then, after all hits and misses are determined, roll one damage roll and apply it to everyone that was hit. If this was indeed the case, you could wait until all attack rolls were made before declaring your immediate interrupt, which would then negate the power in its entirety.
However, when trying to confirm this with the rules, I found out I was wrong.
This part is kind of clear: Rules Compendium page 106 says that when a power has multiple targets, the user of the power chooses the targets. So all targets of a power must be declared before any dice are rolled. Presumably, all targets are declared simultaneously, such that you can't interrupt the person while they are naming the targets; "you are targeted by an attack" would trigger after all targets are declared but before any rolls are made.
However, this is not made clear in the books...
Making Attack and Damage Rolls
This is where things get a bit confusing. Rules Compendium page 214 gives the order of execution for attacks: (1) Choose Power, (2) Choose Targets, (3) Make Attack Roll, (4), Check to see if Attack Hits, (5) Deal Damage and Apply Effects.
The book then specifies a 6th step: if the attack has multiple targets, repeat steps 3 through 5.
However, the rules compendium, on both pages 104 and 105, states that attacks with multiple targets make separate attack rolls per target but only one damage roll.
The trick is in the wording of step 5, which does not say to make a damage roll but to apply whatever effects of a power happen on a hit.
In any case, this wording suggests that the single damage roll that is made for an attack with multiple targets is made after one of the targets is hit, not after all the attack rolls are made.
Resulting Order of Execution
- Declare the power, such as a close burst 1 that hits all enemies.
Declare all targets, in this case lets say A, B, and C, where you are target B.
-- Here is where you could declare an immediate interrupt triggered by "you are targeted by an attack." --
- Make the attack roll against A.
- Check to see if the attack hits A.
- Make the Damage roll, apply damage and effects to A.
- Make the attack roll against B (You).
Check to see if the attack hits B (You).
-- Here is where you could declare an immediate interrupt triggered by "you are hit by an attack." If it voids the attack, stop execution here. Note that A already took damage and effects, only B and C are spared from damage and effects at this point. --
Using the previous damage roll, apply damage and effects to B.
-- Here is where you could declare an immediate interrupt triggered by "you take damage." If it voids the attack, stop execution here. If it is an interrupt, you don't take damage; if it is a reaction, you still take damage.
- Make the attack roll against C.
- Check to see if the attack hits C.
- Using the previous damage roll, apply damage and effects to C.
According to this interpretation, you could not stop ally A from taking damage and you couldn't know if C would be hit or not before having to decide if you want to interrupt the attack.
Obviously, a crafty DM would pick the player with the interrupt as the last target of the attack!
Because this is a somewhat convoluted problem of order of execution, I would strongly recommend bringing this situation up with your DM and your party and agreeing what rule interpretation works for your group.
I would personally stick with my original interpretation of making all the attack rolls and then pausing to allow the players or DM to declare attack-triggered interrupts before moving on to damage. My own sense is that there is a "target" phase, then a "target interrupt" phase, then an "attack" phase, then an "attack interrupt" phase, then a "damage" phase, then a "damage interrupt" phase, where everything happens to everyone at the same time each phase.
This is much easier to execute in actual dice-based gaming, where we assign a d20 to each target and roll them all simultaneously, and it provides clear windows for players and DM to announce interrupts. However, this interpretation is strictly incorrect by the rules as written in the Rules Compendium for D&D 4e.