In an alternate reality the Roman empire never fell apart and eventually conquered the world. By the year 1749 the atomic bomb had been invented. Cloning was developed a few years after, this had catastrophic results. The first experimental version created an alien being of sorts which developed a kind of super intelligence as well as seemingly psionic abilities which it could use to make people do whatever it wanted. It also possessed a unique hatred towards it's creators. This alien then proceeded to completely destroy the planet with their own nuclear arsenal in the year 1809.

The story of their empire begins anew in 2100,after they had adapted to the radiation present on Earth and had once again built a thriving society. In the next 100 years their hatred towards other sentient species grew even worse when another alien civilisation visited their planet.These aliens wanted only to study them, however this meant abducting members of the Roman society which caused rising tensions between the two groups. Total war broke out between them and they managed to destroy the invaders' main ships which led the aliens to evacuate their research teams. They then reverse engineered the alien technology they had left behind in the form of a starship.

With their place in their solar system solidified they began looking farther into space. They expanded to and beyond Barnard's star ,which would later become a major shipyard system. Their first alien encounter was with a very ancient civilisation.
This civilisation, luckily for them, was a very friendly one and left them to their own devices. It wasn't until 30 years after 2200 that they found another civilisation on their tech level. This was a robotic empire( Gestalt Consciousness), more precisely a rogue defence system , which had destroyed their creators and named themselves the "Mechazur Index".

They warred with these robots for years before they made contact with another civilisation. This civilisation was a very peaceful, and prosperous, one that inhabited the rest of their ring of the galaxy. These people were not warlike and as such were no match for the strength of the Romans and quickly fell as they claimed the rest of the ring. This was a great boon to the Romans as two of the planets these creatures had settled on held betharian stone sources which supplied the Roman war machine and gave them the strength to defeat the Mechazurs. However after the defeat of the robots at their capital planet a new federation formed at the Roman Empire's opposite front. It was not long before this federation realised that the Romans' strength was diminished after such a climactic battle and used this opportunity to occupy and eventually seize a lot of their western systems.

In the years that followed the Roman Empire settled many planets in their own territory and developed their economy, as well as their military. They had built up enough strength to reclaim their western systems and so they did, however the council was unwilling to go for a full annexation, reasons given were that the people would not support prolonged fighting. After many more years another empire rose to power in their sector, also eradicators like the Romans themselves, and destroyed most of the federation which was now known as the Praathi Star Alliance. These exterminators ,a.k.a. Thembolan exterminators, were still no match for the Romans however as they were now the second strongest empire on the galaxy after the ancients. After a few more years they discovered wormhole technology and stabilised a wormhole that led directly to the heart of Thembolan territory.

This started a war of gargantuan proportions, which lasted for about 4 years and resulted in an overwhelming victory for the Roman Empire. After this victory the Romans started destroying federations and alliances of weaker states until they went to battle with the ancients. The final battle with the ancients was a very dramatic one. It was fought over the home world of the fallen empire, a Gaia world of unparalleled beauty, with fleets of almost equal strength. Fortunately the Romans brought a small fleet with a Titan flagship just in time to change the course of battle and they won a triumphant victory.

After having achieved control of Fallen empire level technology they were unstoppable and destroyed the endgame crisis (unbidden) easily .They then built a ringworld in the center (or the nearest I could get it) of the galaxy to signify their dominance.

The Galaxy was theirs.