Lord marshal clothing is obtained from the Mobilising armiesminigame by spending reward points. They are fully cosmetic, and therefore give no stat bonuses. They can be purchased from Lord Marshal Brogan, found on the second floor of the Officer Tower.
The Lord marshal set is highly praised and rare among players as only around 16,000 players have reached the rank required to wear it.  There is a limit of 12 ranks gained per hour, so it would take at least 25 hours of straight game play to receive the required rank. The total set costs 802,900 reward points (assuming only one of either the trousers or the skirt is bought). Players can store the full set in the Magic wardrobe of their Costume room.