The White Sash Battalion was a sub-section of the Imperial Guard during the reign of the Emperor Tortaalik. They were led by Captain Lanmarea at the beginning of the Phoenix Reign, and had a certain rivalry with the Red Boot Company under Captain G'aereth.
The company originally had the role of protecting the Emperor, but after the events of The Phoenix Guards, Captain Lanmarea was dismissed for her role in those events, and G'aereth was promoted to Brigadier. It was his decision to then form the White Sash Battalion into what could be considered the equivalent of a police force.
Brigadier G'aereth later placed Baroness Stonemover in charge of the battalion, and she held that position until Adron's Disaster destroyed Dragaera City, and all of the Imperial Guard save for Khaavren.