Interesting thought. Ysera is said to be one of the last spirits to enter Ardenweald before death broke/the anima drought. So that narrows the timeline down to sometime after Val'sharah but still during Legion. You know what would fit with that? Stormheim... 
Well, Vol'jin died before Ysera and he had the whole thing of being saved by an unknown entity from Mueh'zala. I'd say Death broke around that time. And that Ysera got special treatment thanks to Elune much like Vol'jin did.
