The Roman Empire is part of the Legacy of Rome DLC. The Byzantine Empire and the Holy Roman Empire (if Byzantine conquered) are the only empires that can create this title. In order to make this title you need to capture several duchies in Italy including Latium, Ferrara, Genoa and Venezia, the Balkans, Asia Minor, Syria (Antioch), Coast of Egypt (Alexandria) and North Africa (Tunisia).
You will also need lots of prestige in order to create the Roman Empire.
Imperial Reconquest can be used on the following kingdoms(not counting Muslims, you use Holy War on them):
- Kingdom of Serbia
- Kingdom of Croatia
- Kingdom of Italy
- Kingdom of Sicily
- Kingdom of Burgundy
- Kingdom of Aquitaine
- Kingdom of France
- Kingdom of Brittany
- Kingdom of Frisia
- Kingdom of Lotharingia
- Kingdom of England
- Kingdom of Wales
- Kingdom of Aragon
- Kingdom of Navarra
- Kingdom of Castile
- Kingdom of Leon
- Kingdom of Galicia
- Kingdom of Portugal
- Kingdom of Andalusia
- Kingdom of Bavaria
Some ways to begin:
- The easiest start date that I've found to do this is after 1076 when the Sultanate of Rum is created and the Fatimids of Egypt are weakened. Ignore the Muslims because now they are most likely busy fighting among themselves at this date. Instead go for Wallachia and Croatia. You should be able to grab parts of Croatia and entire Wallachia (just ignore the penalty since they are pagans so anyway it doesnt matter much if they dislike you). Next,move into Italy and capture Venice. Then head south and capture Apulia province. If there are still muslims on Siciliy you may want to consider attacking them at this date. Also keep an eye on the HRE and the two Seljuk kingdoms in the east. If any province in Italy breaks away from the HRE annex them using Imperial Reconquest casus belli. Imperial Reconquest can no longer be used to form the empire, only after the reformation.
- As an Orthodox player, fabricating claims in Latinum and Mending the Great Schism is a great way to grab the territory required from the HRE, providing the Kaiser is of the appropriate religion for you to Holy War against. An excellent starting position is as the Duke of Apulia in 1066 or anytime before the Kingdom of Sicily is formed. When you are ready to create the Kingdom, Swear Fealty to the Basileus. As a Catholic you will be able to go on Crusade, which may provide with required territory and Piety to safely convert to Orthodoxy. The Duchy of Tunis is also quite close and a perfect place to move your capital, as a Duchy it is the second best in the game. Personally I rename some of the provinces, Nova Rome, etc. Further Restoration and Expansion.
- With Old Gods installed, 867 becomes the perfect date to start, as you begin strong, and your nearest possible enemies are weakened. You should seize Muslim territories whenever possible using Holy Wars against Muslims, and De Jure claims against Christians; do it fast, before the Muslim Empires reunite again. Restore the Varangian Guard as soon as possible (they are maybe the most cost-effective mercenaries in the game, and only the Byzantine Emperor can use them) and use them as first invasion wave. The Brotherhood of Holy Sepulchre will help against the Pagan and Muslims, if you can afford it. One trick is to declare multiple wars against weakened enemies, or to press multiple fabricated claims, so you don't have to raise levies again; another is giving excess land and titles to your heir (even kingdoms), as he almost always will have a good opinion of you, and provide a great amount of levies. While you conquer all required Muslim Duchies, use marriages and heritages to seize the Kingdom of Italy (and possibly Bulgaria); also, mending the Great Schism may help with external diplomacy. After 150 years, with aggressive expansion and good vassal and bloodline management (Elective is the best for that), you can have all the required duchies.
After creating the Roman Empire you have access to new events involving the restoration of Rome's borders as they were under Trajan and even expanding further. The events arn't listed in game but the player will recieve an announcement when they have achieved one. Events and Requirements can be found in Roman_Events.txt found in \Steam\steamapps\common\Crusader Kings II\events\Roman_Events.txt
When you create the Roman Empire every new emperor or empress gets the trait Augustus (ruler of The New Roman Empire), which increases opinion by +15. Also, the first emperor/empress has his/her nickname changed to "Glorious". You also get the Imperial Reconquest casus belli for further expansion, valid for the above kingdoms, and the option to change your capital to Rome.
From now on, you should be able to expand and consolidate the Empire on your own. Beware of invasions from the East (and Aztecs if you have Sunset Invasion installed), revolts, factions and Jihads/Great Holy Wars, especially after succession.
You can cheat to get the Roman Empire, simply press "~" or alt + 21 and type :
"give_title e_roman_empire <char-id>"
To find your char id you need to type "charinfo 1" and then hover your mouse over your ruler's portrait.Window will popup containing debug information,including char id.
You'll still have all your old titles, and if you use this cheat has the byzantine emperor it's de jure land will not instantly change to Roman Empire.