History of Europe
In 395 Theodosius I, who had made Christianity the official religion of the Roman Empire, would be the last emperor to preside over a united Roman Empire. The empire was split into two halves: the Western Roman Empire centred in Ravenna, and the Eastern Roman Empire (later to be referred to as the Byzantine Empire) centred in Constantinople. The Roman Empire was repeatedly attacked by Hunnic, Germanic, Slavic and other “barbarian” tribes (see: Migration Period), and in 476 finally the Western part fell to the Heruli chieftain Odoacer.
Holy Roman Emperor
From an autocracy in Carolingian times (AD 800–924) the title by the 13th century evolved into an elected monarchy chosen by the prince-electors.
Various royal houses of Europe, at different times, became "de-facto" hereditary holders of the title, notably the Ottonians (962–1024) and the Salians (1027–1125). Following the late medieval crisis of government, the Habsburgs kept possession of the title without interruption from 1440–1740. 
The final emperors were from the House of Lorraine (Habsburg-Lorraine), from 1765–1806.
The Holy Roman Empire was dissolved after the defeat at Austerlitz by emperor Francis II, who continued to rule as Austrian emperor.