Visit The knight, Sir Edmund in the castle, if he says something blink around the castle until you land in the reinforced vault. There will be a convoluted key and some gold on the floor, take only the key. Return to Edmund and give him the key to obtain the mark.
WARNING You will be teleported out to prison when you get to the reinforced vault so do not take too long picking up the key!
For exhibiting integrity and fealty, the Powers have granted thee a Mark. You have been granted the Mark of Fealty! You gain 47 practices!