How much damage does mind spike do against an AoE type attack?

Question

My players are fighting a dragon who naturally has a breath attack, which is a close blast.

The defender in this case is a battlemind, having the dragon marked with his Battlemind's demand.

The marked dragon who is adjacent to the defender uses his close blast 5 attack on the allies of the defender without including the defender in the attack.

Mind Spike
At-Will + Force, Psionic, Psychic
Immediate Reaction Melee 1
Trigger: An adjacent enemy marked by you deals damage to your ally with an attack that doesn't include you as a target
Target: The triggering enemy
Effect: The target takes force and psychic damage equal to the damage that its attack dealt to your ally.

Does the defender deal the total damage dealt to all of his combined allies by this attack, or only a single ally? How does he choose? Note: The target is the enemy, not the ally.

Answer 1

Only once.

The key is "deals damage to your ally". Mind Spike will trigger once for each ally, however, the defender only has one Immediate Reaction to use. Technically, the defender can choose which ally getting damaged will trigger his Mind Spike. That won't make much of a difference most of the time.