After you complete the final mission with the House Harkonen, the Emperor speaks of releasing the Saudukar in the FMV sequence that follows. These Saudukar are the infantry units that make up most of the Emperor's light infantry. They are the ones that wear the helmet with the black "visor". To be able to use this unit, you have to attack the Emperor first on the last mission with any house. The Emperor is the purple base/fleet. Bring an engineer to the battle when you attack him. After that, take control of the Emperor's palace. You can now build Saudukars from your barracks.