In order to strengthen the authority of the high duke, it is believed that Agnes took part in the downfall of one of the most powerful nobles in the country, the voivode Piotr Włostowic, who supported the junior dukes. According to a legend, the capture of Włostowic was thanks to Agnes, because she sent her own retainers to his castle and they captured him during the night. This event was recorded in German contemporary historiography; however, since this story is not confirmed, it not generally accepted by modern historians. Agnes demanded Włostowic's death, but her husband decided instead to make an example of him. Włostowic was blinded, muted and sentenced to exile.