- +5% Discipline
- +1 Yearly legitimacy
- −1 National unrest
- +2 Diplomatic reputation
- −20% Stability cost modifier
- +15% Land force limit modifier
- +10% Infantry combat ability
- +20% Production efficiency
- +10% National tax modifier
- −20% Core-creation cost
- +33% National manpower modifier
Historically, the Roman Empire was the dominant power in Europe in the first few centuries AD. While the Western Roman Empire ceased to exist long before the era covered by Europa Universalis IV, the Eastern Roman Empire continued to survive as Byzantium. Any Christian or Pagan nation can restore the Roman Empire by decision after re-conquering most of the historical Empire. The other way to play the Roman Empire (and, before patch 1.16, the only way) is to restore the Roman Empire as Byzantium in Crusader Kings II and converting the save to EU4.
Restore the Roman Empire
Please help with verifying or updating this infobox. It was last verified for version 1.16.
Roman Empire does not exist.
The Roman Empire does not exist at any start-date, and the formation requirements are particularly onerous. While forming it is possible as any Christian or Pagan, it will be relatively easier for large countries in the Mediterranean such as Castille or France. Conquest of the northern half of Italy should wait until after the Shadow Kingdom event takes it out of the HRE. As a Catholic, it is prudent to wait with taking Rome itself until all other requirements are met. It might be in order to note that the islands of Azores, Madeira and Canary Islands are part of Iberian region and are therefore needed to form the Roman Empire. Same goes for tiny islands in Aegean Sea, which are part of the Balkan region.
Consequences of forming the Roman Empire
The Roman Empire is specifically exempted from the penalties Catholics get for owning Rome. Forming the Roman Empire switches the primary culture to Roman. It is impossible to switch back using the Accept Cultural Shift decision. Because of the large areas that need to be owned to form the Roman Empire, it is likely that the original primary culture will no longer be an accepted culture. This may result in income loss. Note that Roman is part of the "Lost Cultures" group, not in Latin, as might be expected.
Importing from CK2
One can export a saved game from Crusader Kings II and import it into Europa Universalis IV with the EU4 Save Converter DLC. To do so, one would have to play as or become the Byzantine Empire in Crusader Kings II, reconquer a set amount of duchies, then select a decision to restore the Roman Empire (with moving the capital to Rome and mending the Orthodox-Catholic Schism as optional decisions). This is difficult, since many of the territories required are far outside the De Jure reach of the Empire. Also, most territories are either held by Muslims who will band together in a jihad to repulse the Byzantines, or Italian city-states capable of hiring large amounts of mercenaries. For a full guide to forming the Empire in CKII, see here.
If the average technological advances in the entire realm passes a certain point, the Empire's technology group will be Western instead of Eastern after transfer to EU4.
Capital, Culture, Religion
The capital, state culture and state religion will be transferred over from CKII. This usually means that either Rome or Constantinople will be the capital, and the state religion and culture, reliant on the culture and religion of the current Emperor, will usually be Greek and Orthodox respectively. While the player can groom the Emperor to be a different Culture or Religion from the education mechanic, since the Religion and Culture of provinces carries over as well, this can cause a higher chance of rebellion and decreased income and manpower. If the Emperor is Catholic but the rest of the Provinces are Orthodox, there is a high chance of Religious Revolts.