Knight-errant

A Knight-errant (plural Knights-errant) was a rare and temporary title and rank given to an member of the Order of the Imperial Knights indicating that he/she have been assigned a mission or task that takes precedence over all their other duties. The Knight-errant must complete the assigned task in order to be able to return to his/her regular rank and duties. Any member of the Order could become a Knight-errant, regardless of rank (at least in theory).

Overview
The assigned mission is referred to as the Knights errand or Knights quest.