User blog comment:IamJakuhoRaikoben/Chapter 457: Naked Battle, Review./@comment-7418318-20151019082615

I know I just said that I don't want to speculate about Layla and Brandish's past, but I guess I just can't keep myself from it after all, but then on the other hand, I'm just going to pick a little something out.

Could Brandish be responsible for Layla's death?

Maybe she shrunk something inside of Layla. If Brandish for example shrunk one or several of Layla's organs, they might underperform. Layla could keep on living, but her body would work less and less well, first resulting in the weakness that prevented her from keeping a mage and utimately resulting in death.

It sounds quite excruciating, but with the way Brandish looked when she learned that Lucy was Layla's daughter I wouldn't be too surprised.