Somehow I don't think so, but I only really used the mantle for 1 fight.
It only went off one time at Jorm (I think it was like an hour and 15 minute fight), and that was def. when my shadows were down and I was in the middle of recast (it saved my life tho so props to it). I could've just gotten unlucky and it just didn't proc when I had shadows up, but I doubt it. If its true that means its a really good piece to always have on.
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