Ok this has been bugging me for ages, we never find out what happened to Sierin between New Spring and Eye of The World. I don't think anyone ever mentions her after New Spring...

Chesmal had members of the Red Ajah kill her. She was Amyrlin from 979-984 and was succeeded by Marith Jaen. We don't know what she actually did but she was very strict. Alviarin mentions that the Black Ajah had nothing to do with the actual murder. Moraine says that she was a Gray but acted more like a Red, even choosing a Red Keeper and Mistress of Novices

Hmmm where was this? BWB?

http://www.encyclopaedia-wot.org/

 

Characters-->S-->Sierin Vayu

oh

My guess is Tsutama did the actual deed. My analysis of it, from my WoT Theories, can be found here: http://www.dragonmount.com/forums/topic/51173-who-killed-sierin-vayu/