Star Wars: Shattered Worlds
These specialized members of the Order of Sith Lords act as trainers for new Sith acolytes and come from the ranks of Sith Lords and their apprentices. Generally they specialize in some area of knowledge, such as prophecy, Dark arts, lightsaber combat, etc. This position was re-instated by the Emperor to establish administration over the newly restored Sith Academy on New Korriban.
Currently there are three Overseers at the New Korriban academy, headed by Lord Cephus, who was appointed by the Dark Council in recognition to his service at the Battle of Corellia in order to retain a use for him after the critical injuries he suffered there.
Overseers are also responsible for assisting worthy candidates in finding Sith Lords to apprentice them, and some who are among the ranks of the Sith Lords themselve may attempt to take students on as their own apprentices.