The Roman Empire is a potential de jure empire consisting of 9 kingdom titles, 40 duchy titles and 122 county titles. The title does not exist by default in the timeframe of Crusader Kings II or The Old Gods DLC, but can be created by the player or the AI. Creating the Roman Empire requires Legacy of Rome DLC.
De jure Territory
The de jure kingdom titles within the Roman Empire are the same as for the Byzantine Empire. By default these are:
Born in the Purple (requires Legacy of Rome DLC) - Children born to the Emperor are 'porphyrogennetos' - literally born in the purple. The term comes from the Purple Chamber of the Imperial Palace, a room traditionally used for the birthing of the children of the Emperor. Children who are born in the purple are placed higher in the line of succession than older siblings who are not. This can be circumvented by giving an older child the honorary title of 'Despot' - they then count as being born in the purple.
Legacy of Rome (requires Legacy of Rome DLC) - The Roman Empire is created by decision, and replaces the Byzantine Empire. Your character must be a Christian (being an Orthodox ruler is not required (though it is recommended), neither is being of Greek or Italian culture) in possession of the Byzantine Emperor's title and fully control the following 14 duchies to restore the Roman Empire: Latium, Venice, Ferrara, Sicily, Genoa, Capua, Apulia, Thrace, Antioch, Jerusalem, Alexandria, Athens, Tunis and Croatia. Restoring the Roman Empire costs 3000 prestige and replaces the Byzantine Empire's name and coat of arms to those of the Roman Empire. Emperors of the restored Roman Empire gain a unique 'Augustus' trait, which increases their prestige and gives a large +15 opinion modifier to all vassals..
Imperial Reconquest (requires Legacy of Rome DLC) - Rulers of the Roman Empire gain the use of the Imperial Reconquest casus belli. This CB can be used against rulers of territory in the old Roman Empire(not working with Muslims rulers), and targets one duchy at a time. The kingdoms considered to be part of the old Roman Empire are: Greece, Anatolia, Armenia, Bulgaria, Serbia, Croatia, Italy, Sicily, Bavaria, Burgundy, France, Aquitaine, Britanny, England, Wales, Frisia, Lotharingia, Aragon, Castille, Andalusia, Leon, Asturias, Galicia, Portugal, Navarra, Syria, Jerusalem, Egypt, Africa, Mauretania, The Papacy and Venice.
Abyssinia • Arabian Empire • Britannia • Byzantine Empire • Carpathia • Francia • Hispania • Holy Roman Empire • Italia • Mali • Persian Empire • Roman Empire • Russia • Scandinavia • Tartaria • Wendish Empire • Rajastan