Before the apparent extinction of the Sith order (a millennia before the rise of the Empire), the Sith Lords had ruled many systems and in some of them were revered as gods. The Servants of the Sith arose as a subservient order dedicated to worship and service of the Sith Lords, carrying out tasks ranging from menial and ceremonial tasks to executions and assassinations. When the Sith were apparently destroyed the remnants of the order went into hiding, passing down their traditions and skills through the centuries and awaiting the prophesied time of their return.
Servants are highly disciplined and fanatical in combat. They are immune to mind tricks and many have limited force skills, though they are never taught enough to be a threat to the Sith.