Magister officiorum

[5] Constantine also transferred the supervision of the agentes in rebus, a corps of trusted messengers who also functioned in a bureaucratic role as monitors of the imperial administration, to the magister.

[a] Perhaps at the same time, senior agentes were appointed as heads (Principes) of the staffs of the most important provincial governors: the praetorian prefects, the vicars of the dioceses, and the proconsuls of the provinces of Africa and Achaea.

[19] The office's powers were further enhanced in the eastern (or Byzantine) half of the Empire in 395, when Emperor Arcadius (r. 395–408) stripped the Praetorian Prefecture of the East of some of its jurisdiction over the cursus publicus, the palace guard (Scholae Palatinae) and the imperial arsenals (fabricae) and handed them to the magister officiorum.

[3][24] At least until the time of Emperor Leo VI the Wise (r. 886–912), however, the full former title was remembered: his powerful father-in-law, Stylianos Zaoutzes, is recorded once again as "master of the divine offices" (μάγιστρος τῶν θείων ὀφφικίων).

[25][26] In his administrative functions, the magister officiorum was replaced chiefly by the logothetēs tou dromou, who supervised the Public Post and foreign affairs,[27] while the imperial bodyguard was transformed into the tagmata.

From the reign of Michael III on, the title was conferred on more holders, effectively becoming a court rank, the highest in the Byzantine hierarchy until the introduction of the proedros in the mid-10th century.

[28] The List of Precedence (Klētorologion) of Philotheos, written in 899, implies the existence of 12 magistroi, while during the reign of Emperor Nikephoros II Phokas (r. 963–969), the western envoy Liutprand of Cremona recorded the presence of 24.