if she's not in limbo, illyana isn't winning this. she can delay and make diana hurt, but that's as far as i would go with it. i know she currently has limbo 'inside her' but until she starts showing mephisto level power while carrying it, it still hasn't translated into her same level IN limbo. spells to shield and cause problems, teleporting to make it tough. the sword would hurt her--maybe badly, but in the end speed, strength and durability would catch up and end illyana. and it's not like ww is unaccustomed to fighting magical specialists. what would be interesting would be seeing her try and deflect the sword with her bracers. they might actually end up broken...