Amazing.
I can only find my DMG at the moment, so I can't double check, but if the power really lets you stun opponents on a miss, it's broken.
By the way, why is the PC trying to miss? What happens if he hits?
Its a level 20 Blood mage daily. If you hit it does 6d6+int mod, causes 10 ongoing psychic damage and stuns the target, save end both effects.
On miss it is stunned until end of your next turn. It is burst 3, 20 range. only hits enemies.
I'm not sure why someone would want to miss since you make saves at the end of your turn so save ends is effectively at least the same as stunned until end of your next turn. Though I could totally be missing something.