Complex sentence: Sheriam was the Mistress of Novices during Siuan's reign as Amyrlin, a reign that ended when the White Tower split and many Aes Sedai fled to Salidar.
Split into simpler sentences: Sheriam was the Mistress of Novices during Suian's reign as Amyrlin. A reign that ended when the White Tower split and many Aes Sedai fled to Salidar.