Reason

The First Elder felt her head swirl as nausea overtook her. She started seeing colors all around her despite the lack of any. Her eyes were bloodshot, with blood leaking from her nose and ears.

Then, they began leaking from her mouth as well.

She collapsed onto the ground, unable to hold herself up. She laid on her back, looking up at her disciple in disbelief.

"You… you poisoned me?" she asked. She couldn't remember when or understand why. Remembering anything was difficult for her at the moment. She didn't even have the strength to get angry at her disciple.

Hei Tingxie removed a ring she wore on her fingers and threw it onto the ground. A single sharp tip stuck out from the side of the ring, with some sort of purple-colored liquid on its tip.

The First Elder looked at it. Only now did she realize how she had been poisoned. 

But why? She still wasn't sure.