The Sevastokrator was essentially a Vice Avtokrator (or Vice Emperor) position in the Empire of Videssos. The actual powers of a given Sevsatokrator were defined by the reigning Avtokrator. For example, Anthimos III had little taste for actually ruling, and so allowed his uncle, Petronas, a great deal of power as Sevastokrator. When Krispos became Avtokrator and defeated Petronas, he did not appoint a new Sevastokrator, opting instead to appoint his foster brother Mavros to the office of Sevastos.
Centuries later, Mavrikios Gavras overthrew Strobilos Sphrantzes. Once he became Avtokrator, he appointed his brother Thorisin as Sevastokrator, but made Vardanes Sphrantzes, the nephew of the deposed emperor, the new Sevastos in exchange for his support.