Official Canon Edit
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 Edit
Miriael is extremely frustrated that nobody takes her seriously. Despite the distinction of being the sole Sister of Battle to willingly join Chaos, she is constantly upstaged by the more commonplace Chaos Space Marines. Her frustration is a major source of pleasure for other Slaaneshi cultists, which only makes her even more angry.
In a video by Eliphas, she is seen torturing and interrogating a captured Inquisitor. Despite this, the Inquisitor openly mocks her, asking what proof she had that she was actually a former Sister of Battle when she could just be a conceited heretic wearing stolen armor.
Miriael 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.