From Halopedia, the Halo wiki
A seneschal is an honorable position among the noble houses of the Sangheili, likely involved in the stewardship of domestic affairs. Currently, not much about the seneschal order is known, aside from its relation to the Evocati. Nevertheless, in the postwar period a number of documents have been recovered providing more insight on the role's purvew. During the time of the Sangheili's membership in the Covenant, those promoted to the rank of Evocatus were expected to take up a number of other roles after some period of service, one possibility being that of a seneschal.