After some testing it does appear that multiple hits by using Magic Missile's 'Split' rune will trigger the effects of the Prodigy passive ability multiple times. With this in mind in conjunction with this thread relating to Electrocute, I believe it safe to state that the 'Prodigy' passive will proc every single time one of the signature abilities hits. This should mean that the Spectral Blades ability is overpowered for Arcane Power regeneration (at the cost of standing in melee range of course).
After some further testing with Spectral Blades I can confirm this is actually how the Prodigy passive works (one attack with Spectral Blades on a group of 7 enemies resulted in my Arcane Power going from 'nearly empty' to 'nearly full' in one attack).
My methodology for testing:
- Act 1 inferno, first quest
- Arcane Orb as my secondary attack
- Prodigy active as one of my passives
- Magic Missiles + Split rune as my primary attack in the first pass
- Run out past Rumford at the gate into Old Tristram Road, you'll meet a pack of 6-8 mobs
- Empty Arcane Power using Arcane Orb
- Attack the group
- Observe results
What I found:
- Magic Missiles without the Split rune will provide less Arcane Power than with the Split rune
- Spectral Blades on a large group will provide Arcane Power back for every enemy that is hit
- Electrocute will provide Arcane Power for targets hit by the chained element of the attack
The Prodigy passive ability does exactly what it says it does. When you deal damage with a Signature ability with the Prodigy passive enabled, you will get 4 Arcane Power per enemy that is damaged during that cast.
For example; in the instance that you're using the Magic Missile ability with the 'Split' rune, you will gain 4 Arcane Power for missile, under the assumption the missile hits an enemy. If each of the 3 missiles hit the same enemy that still counts as three hits and you will gain 4 Arcane Power per hit accordingly.
This stacks with the equivalent runes on the Signature abilities themselves, so you can use Prodigy and the Siphoning Blades rune for Spectral Blades, for example.