Also known as a Jhariuk, the Harrowmaster was a leading commander given overall control of Alpha Legion forces in any given theatre of operations. It was the task of a Harrowmaster to keep track of the encompassing shifting events in a warzone down to the smallest detail and shape the actions of his own forces accordingly. The position of Harrowmaster was extremely prestigious within the Alpha Legion, and one of the few where the holder of the title was known publicly.
Like most things with the XX Legion, it is unknown if the title of Harrowmaster still remains officially in use as of M41. However, Quetzel Carthach, leader of the Sons of the Hydra Warband continues to make use of the title. 
- 1: The Horus Heresy Book Three - Extermination, pg. 90
- 2: Sons of the Hydra (Novel) - The Serpent's Egg