The Eternal Imperium was a vastly powerful empire controlling a large portion of the entire Omniverse and incorporating many different forms of life. It was formed from the ashes of a huge number of empires which had obliterated each-other in one of the most horrible wars in the entire Omniverse. At its peak, it was the 2nd most technologically advanced group in the Omniverse and the 3rd most powerful, next only to The Mevalkagna and the Idea Breakers. Considering the most technologically advanced group was a fraction of the size of The Eternal Imperium, it may seem strange that it was not the most technologically advanced, however The Eternal Imperium had a strong belief in quantity over quality. They believed huge numbers of weak weapons and poorly trained soldiers would always triumph over a small number of elite soldiers with powerful weapons. It was this methodology of strength through numbers that allowed them to survive the great wars which preceded them, and allowed them to grow across the Omniverse.
The Eternal Imperium was a deeply authoritarian state which oppressed and murdered its people constantly. This would ultimately lead to a revolution provoked by failure in a war against The Mevalkagna. After this revolution, the people of The Eternal Imperium would go on to create 2 new massive civilizations: The Great Cosmic Republic and the Eternal Legion.
The Eternal Imperium created some of the most advanced technology within the Omniverse, though little to none of this was shared with the people and was instead reserved for the authorities and the bureaucracy. They created impressive forms of easy communication and transportation across the many different Archverses within their dominion as well as many advanced super-weapons to fight other powerful empires and entities. It should be noted these super-weapons were far less advanced than they should have been as The Eternal Imperium consistently turned down designs for super-weapons that could not be easily mass-produced. They also created prototypes for ships that could travel beyond the Omniverse however they were unable to achieve success.
The most advanced of their technology was destroyed by The Mevalkagna during the collapse of The Eternal Imperium. However, some of the weaker technology, such as the Guardians, survived. Much of this technology was collected by the Eternal Legion and The Great Cosmic Republic, which greatly aided their expansion programs.
A huge number of extremely powerful civilizations throughout the Omniverse had become inter-locked in a brutal conflict in which the laws of time itself were pushed to their absolute limit as each empire found new ways of causing horrific destruction through paradoxes. The paradoxes and time loops involved in this war were even worse than those found within the Negative First Causal War and there seemed to be no chance of escape. Eventually, The Eternal Imperium rose up and sought to end the war. They declared themselves 'Eternal' as they believed the true cause of all destruction was time, as it was the manipulation of time that had caused simple disputes between empires to last forever, looping over and over again in ways that strayed further and further from linear time. Thus, The Eternal Imperium sought to be beyond time. They chose to end the war through a brutal genocide of all involved within the war, killing every single entity. Due to their lack of strength in any way except numbers and perhaps their ontokinetics, many of their own also perished. This horrific genocide was known as 'The End of Time'. After the genocide was complete, they retroactively changed the laws of causality, using their advanced technology, which would otherwise be impossible, and thus ensured that it was impossible for the events to play out in such a way that anyone besides them could survive The End of Time. From there they sought to expand their empire and began a conquest over huge portions of the Omniverse.
The brutal oppression of the citizens of the Eternal Imperium obviously made many wish to rebel, however as with much of human history, successful revolutions cannot happen without an inciting incident which either weakens the dictatorship or inspires the people.
This incident was the war with The Mevalkagna. The Mevalkagna believed that The Eternal Imperium was growing too powerful and sought to erase it. While The Eternal Imperium held them off at first, they eventually began to be overpowered by The Mevalkagna as they found their own methods of strength through numbers being used against them. This inspired the people to rebel, and they were able to completely tear down The Eternal Imperium from the inside. The Great Cosmic Republic was formed in its place and prepared to fight off The Mevalkagna, however, The Mevalkagna simply destroyed all of their most advanced technology and left, as the new Great Cosmic Republic was no longer enough of a threat to be worth exterminating. The Great Cosmic Republic sought to work to benefit their people more than their rulers, at the expense of growing in power, causing those who wished to expand to break off and form the Eternal Legion.