MAGISTER OFFICIORUM

In Late Antiquity, the Roman position of 'magister officiorum' (lat. literally: "Master of offices") can first be traced to the rule of Roman Emperor Constantine I, but may have been first established by Diocletian.
The ''magister officiorum'' was the most powerful civilian official besides the praetorian prefect (''praefectus praetorio''); he was the organizational commander of the agentes in rebus and had authority over the public postal system as well as a number of officers at court. He was charged with the supervision of the imperial arsenals, and since the 5th century the ''limitanei'' also fell under his authority, at least in the Eastern Roman Empire; as a civilian he did not, however, directly command them. He was also the commander of the palace guard.
In the course of time, the office also took over the coordination of foreign affairs (already in the late 4th century, the official translators and interpreters were under the control of the ''magister officiorum'' for this reason); one of the most important incumbents of this office was Petrus Patricius, who held the position from 539 to 565 and in this role undertook numerous diplomatic missions for Justinian I.