Short answers, pulled from the story.
The Roman Empire formally began in 27 BC when Octavian and the Senate concluded a political agreement known as the First Settlement. This event marked the start of imperial rule under the title princeps, meaning first among equals.
Diocletian introduced reforms in 285 AD that transformed the empire into an absolute monarchy called the Dominate. Emperors adopted the diadem crown as their supreme symbol of power and abandoned the subtle republican pretenses of earlier centuries.
The Augustan institution lacked a clear succession system, forcing emperors to adopt candidates as sons rather than announce heirs formally. Legitimacy depended on acceptance by the Roman army rather than any legal criteria or primogeniture rules.
Constantine the Great moved the capital from Rome to Constantinople in 330 AD, formerly known as Byzantium. He became the first Christian emperor and established the idea that rulers were God's chosen protectors on earth.
Constantinople fell to Ottoman Turks in 1453 AD when last emperor Constantine XI Palaiologos died in battle. Morea and Trebizond fell in 1461 AD, marking the final extinction of Byzantine imperial authority.