For the hired servant, see Servant.
The Masked Servants were "vagabonds, street scum, prostitutes..." who were rounded up by the City Watch and made into slaves by Mechanist mind-control technology. Utterly loyal and never needing rest nor feeding, they were superior to normal human servants of the City. However, their true purpose were as weapons developed by Karras in order to destroy organic life. They were given to the Nobility, those with big gardens, as gifts. The Servants would be recalled by Karras' Guiding Beacon to have the cultivator and rust gas installed, then returned and activated.
Another type of Servant can be encountered inside Soulforge Cathedral. When Manus asked Karras why he insisted upon making more Servants, suggesting that devout Mechanists would be more proficient, Karras came up with the idea to turn other Mechanists into Masked Servants (Masked Guards in DromEd), that would be far more loyal and pure in the eyes of the Master Builder than normal humans could ever be. This decision came late in the story, as can be seen by the shoddy transformation. Their teal hoods appear to be clamped down by the top corners of the masks and blood can be seen running down their chest armor. It is likely that they were given no choice about the matter, as they can be heard to beg Garrett to kill them and, when slain, thank him.
The process of making the Servants, as well as the subjects condition (alive, dead, or re-animated) is unknown. The urchins kept at the Servant Laboratory in the Eastport Mechanist Seminary are all dead, whereas the bloody uniforms of the Masked Guards suggest that they were alive when operated on. They all appear to have two personalities; one which is a remnant of their former lives and memories (constant mumblings that they are cold, soft crying, and begging for death) and a controlling personality that inhibits their free will, making them seek assistance from armed guards when anything out of the ordinary occurs. This personality could well be connected to the masks or to the Signal Towers.