It worried Fexiryn at how much power Criss had...shi knew all too well the burdens of vast power...only it was so much harder for hir.  Criss wasn't aware of it, but that sort of power could overwhelm a person.  Hir own power shi knew, could have probably destroyed the chest, but shi risked losing hir mind to the thralls of her other side if shi were to ever unleash that much power.