I think it could be interesting, but needs more development. I mean I could just spam this spell from first over and over again and no one could beat me.
Maybe this convert should only work with certain classes of damage, or from certain specific schools? Maybe it should be random what school it is specific to?
What I like best about your idea is that it forces opponents to vary their attacks and think them through, instead of relying solely on giant shadow criticals.
. With spirit armor you get a Negative damage 
200 = 200