Seldaar

I believe the theory is she got the sword when her mom died when she was young. As the flashback we've seen talks about her mom's body being turned over to hide the eyes, and her dad escorting her away from that body and the body of a bloody man. My thought is she killed her dad with the Blade by accident somehow, and also broke the Soulcaster. So she potentially killed both her parents with the Blade, which is why she has a mental block on summoning it.