I originally had it once per short rest, but that felt to weak. Maybe I should make it a recharge power. When I get more time I will check the numbers and make adjustments as needed.Every round a 320 AOE spike seems a bit off. I get how epic it feels, but half of the point of legendary actions is to boost outgoing damage/threat without it being focused all at once, so PCs can respond to the hazard.
I guess a d10 HD character with 16 con has 160ish base HP. Add a heros feast for 11, aid for 20, 10 temporary HP, a death ward and potions of every elemental damage resistance, and they can survive a round of that spike damage. Then you just put her down before she does it twice?