Rator III, also known as Rator Prime, was the third planet of the Rator star system, and the capital of the Romulan Star Empire following the destruction of Romulus. Rator III and IV formed a twin world, much like Romulus and Remus. (TAS episode: "The Survivor", STO - Romulan Mystery mission: "Preemptive Strike")
Rator Prime was the third planet in the Rator system, itself part of the Hyralan sector and the Psi Velorum sector block, located in the Beta Quadrant. Neither Rator III nor Rator IV possessed moons but orbited their sun together. (STO - Romulan Mystery mission: "Preemptive Strike")
Rator Prime's surface was largely covered by a continuous landmass, broken up by lakes and seas of various sizes. The continent itself was covered by a dense forest. (STO - Romulan Mystery mission: "Preemptive Strike")
The Rator system was part of the Romulan Star Empire since before 2151. (ST reference: Star Charts) In 2388, after the destruction of Romulus, the leaders of Rator III attempted to proclaim themselves the new capital of the Romulan Star Empire. The planet's leadership council calling themselves the new Romulan Senate. Other worlds made similar claims, challenging the leaders of Rator III.
2391 saw Rator III become the first planet whose people claimed Admiral Taris to be the Romulan leader, after a military victory of Taris's in the Zeta Pictoris system. The admiral attempted to convince Romulan colony world leaders to come to Rator III in order to build a new government and capital.
By 2409, the capital was known as Nova Roma and the seat of Empress Sela. The Tal Shiar kept a fleet of starships in the vicinity and protected the planet with turret emplacements. When Starfleet learned of plans to attack Vulcan with subspace weapons, SI dispatched a team, led by Lieutenant Tess to destroy the weapon caches, and a fleet to destroy the Romulan assault fleet. Another Federation starship's away team rescued Tess's team when it was captured by the Romulans and completed the mission before joining the Federation fleet, led by the Jupiter-class dreadnought USS San Jose. A battle ensued at Rator IV. The Romulan fleet and its flagship, the IRW Decius, were destroyed, and the Romulans were forced to scuttle their attack plans against the Federation. (STO - Romulan Mystery mission: "Preemptive Strike")
Later that year, the kidnapping of Empress Sela by the Iconians resulted in a struggle for leadership in Nova Roma, and the city descended into chaos. (STO - Romulan Mystery mission: "Cutting the Cord")