The Sevastos was essentially the prime minister of the Empire of Videssos. The role of the Sevastos was to help the Avtokrator rule and, when needed, to command armies for him. They sometimes served as diplomatic envoys, usually to hostile nations, like Makuran.
The position was the second most powerful after the rank of Avtokrator of Videssos. Indeed, most emperors made their friends or supporters or usually somebody they could trust sevastos. If a sevastos did not have sufficient trust of his Avtokrator, he would also be heavily watched.
Krispos, one of the most powerful emperors of Videssos's history, employed his foster brother Mavros as Sevastos after rising to power. After Mavros died fighting Harvas, Krispos replaced him with Iakovitzes,who was known for his sharp tongue and sly mind.
The Videssian rank of Sevastos is based on the Byzantine Empire's equivalent, Sebastos, which is pronounced as sevastos.