Miriael Sabathiel

A sign of strength for the Adepta Sororitas is the fact that in their entire history only a single sister, Miriael Sabathiel, has willingly sought out the power of Chaos. She has turned from the Emperor's light to Slaanesh worship, and now serves as one of the prince of chaos' greatest warriors.

Miriael now wanders the galaxy, slaughtering the enemies of her god. It was upon her daemon-tainted blade that Khorgath, Champion of Khorne, choked to death on his own blood. It was her bolt pistol that executed Justicar Tankrius of the Grey Knights, and her machinations that trapped Autarch Eliathon of Ulthwe and teased his soul from its stone, an agonising process that took a day and a night. Slaanesh has not been idle with his new champion, sealing her armour with daemonic wards, and gifting her with a glowing runesword. It is her siren's beauty that is her greatest strength, however, combined with the ability to see the bare souls of her enemies, and she has corrupted many more Sisters of Battle as well as countless other Imperial Savants.

A full Mission of Sisters of Battle from the Order of the Argent Shroud vowed on Ophelia VII to capture Miriael. All of them swore themselves Slaanesh after being overwhelmed by the unholy will of Sabathiel. These fallen sisters now form her retinue. Inquisitor Tosak set out to hunt her down in the Sactifiza System. He was tortured to death in the bowels of a shattered hive, bereft of his faith. A squad of Black Templars cornered her in Tancred VI, but fell to their knees before her, and gave themselves to Slaanesh. An entire shrineworld fell to Slaanesh through the machinations of Miriael Sabathiel.

It will not be long before Miriael's warband of fallen Sisters, Chaos Space Matines and other, lesser renegades is enough to challenge the most powerful Slaaneshi warbands, those led by former Emperor's Children. The cold fervour that she applied to her life in the Adepta Soroitas is now applied to her worship of Slaanesh. Miriael dreams of the day she can lead a cruel mockery of an Imperial Crusade, and sacrifice all unbelievers to the Dark Prince.

Source

 * Codex: Chaos Space Marines (3rd Edition, 1st Codex)
 * Codex: Sisters of Battle (2nd Edition), pp.18-19, 35