Miriael Sabathiel is a former Sister of Battle, and the first known sister who willingly joined Chaos. She is a champion of Slaanesh.
Official Canon[]
Not much is known about her or why she willingly joined Chaos, but every Sororitas and Inquisitor that has been sent after her has ended up dead or corrupted; a fair few have been tortured to death as well. She wishes to lead her own Black Crusade, but she is a relatively obscure Chaos Champion; whether that is intentional or just an oversight of the writers due to barely writing anything about her is unknown.
TTS Canon[]
Miriael is extremely frustrated that nobody takes her seriously. She appeared as a guest on Lucius and Ahriman's Slaaneshmas special. At first she was confused and insulted by Lucius strange antics, until Ahriman told her that it was Slaaneshmas. Embarrassed that she almost forgot, she doned a Santa hat and quickly joined the fray.[1]