As Cybernetics improved over the years, the applications became a lot more ambitious than replacing a single limb or eye. A Servient is a sapient cyborg, created to serve a single purpose. Many were once slaves, who were bought and "upgraded". Most are designed for work, however some are designed for personal protection.
As they are largely immune to breathing in spores and can work much longer hours than any other race, they are put to work in the mines of The Below.
The first Servients were created in Anno 669 NP. These early prototypes were inefficient, and did not function as well as other workers. It was not until Anno 753 NP that the design was perfected.