The Romun Empire is a large empire controlling large parts of the Eresian continent and the Medo Sea, spreading as far as Xandria. It's seat of power lies in Romn, the empire's capital city. Adol Christin's interests tend to clash with the Romun Empire, often leading to violent encounters between Adol and representatives of the empire. As such, the Romun Empire can be seen as one of Adol's main recurring enemies, alongside the Clan of Darkness.
In Ys: Memories of Celceta, Adol, while still trying to recover from amnesia, has to hunt down rogue Romun military officers who are trying to kill all but the strongest humans. Griselda, a Romun princess who is the governor-general and ruler of Celceta, backs Adol in his quest to dispose of these rogues.
In Ys: The Oath in Felghana, Count McGuire, a nobleman from the Romun Empire who was appointed as the feudal lord of the area, closes the mine which is the main source of income for Redmont and levies high taxes.
In Ys VI: The Ark of Napishtim, the Romun Empire's army lead by Admiral Agares and Ernst conquers Canaan and enslaves several Rehdans on the basis that they are "subhuman". Adol worked with Baslam, Ladoc, and the remaining Rehdans to rescue the hostages and retake Canaan. Ernst also took over the Ark of Napishtim, but was stopped by Adol. Since the Ark's control key Almarion was destroyed in the fight, Adol had to destroy the Ark in order to prevent it from causing destruction.
Analogue to Real WorldEdit
It's an analogue to Roman Empire in real world.