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Year 423 (CDXXIII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Marinianus and Asclepiodotus (or, less frequently, year 1176 Ab urbe condita). The denomination 423 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- August 15 – Emperor Honorius, age 38, dies at Ravenna of dropsy, perhaps pulmonary edema. With no children to claim the throne, Joannes, primicerius notariorum ("chief notary", head of the civil service), seizes the throne of the Western Roman Empire, and is declared emperor. Among his supporters are Flavius Aetius, Roman general (magister militum). Joannes' rule is accepted in the dioceses of Gaul, Hispania and Italia, but not in Africa.
- Winter – Emperor Theodosius II refuses to recognize Joannes as emperor, and prepares for war. He mobilizes an expeditionary force under command of Ardaburius, and his son Flavius Aspar.
- Theodoret becomes bishop of Cyrrhus (Syria). He converts more than 1,000 Marcionites in his diocese.
- Smith, Sir William (1849). Dictionary of Greek and Roman biography and mythology. Vol. 3. C.C. Little and J. Brown. p. 1211.
- Urbainczyk, Theresa (2002). Theodoret of Cyrrhus: the bishop and the holy man. University of Michigan Press. p. 21. ISBN 978-0-472-11266-1.
- "Eulalius | antipope". Encyclopedia Britannica. Retrieved March 29, 2020.