From the Setting Guide
Mendax is the herald of Pravus The Corruptor [NE] and focusses on physical corruption. Over the course of decades, Mendax gave power to their followers in the city of Mornburg in exchange for corrupting their physical bodies.
Eventually, the cult grew strong enough to storm the government buildings of Mornburg and take control of the city. The gates were sealed and the population was dragged from their homes to be corrupted.
It took over a year of concentrated effort, during which time the order gained more and more power within Aeldal, but eventually the rebellion was stopped at Mendax's Reckoning.
Since that time, the Order of the Silver Gauntlet has remained commissioned by the Aeldal state. The law of the lands actually names the captain of the company as military dictator for the region.