The Eternal Legion is a powerful empire that was once seeking to rule the Omniverse and potentially other Omniverses after finding out The Omniversal Traveler may have exited the Omniverse through similar ways that they've been trying to, and that he may have found other forms of existence. Our version of the Eternal Legion was erased by an A.I. they created known as Terminus and so obviously failed in their attempts to expand beyond the Omniverse.
An alternate version of the Eternal Legion from a different Monocosm succeeded and created an empire beyond the Omniverse, however through a long chain of events was ultimately erased and unknowingly created an unimaginably powerful being known as Venrensorys.
The Eternal Legion was one of two civilizations formed from the remains of the fallen Eternal Imperium, the other being The Great Cosmic Republic. While the latter civilization focused inward, seeking to facilitate the best possible living conditions for its population, the Eternal Legion looked outward, making it their primary goal to conquer the Omniverse and, once existence Beyond the Omniverse was discovered by the Omniversal Traveler, they shifted their goals to exiting the Omniverse and conquering all that was beyond it.
Their plan to do this was to burn the entire Omniverse and convert it into power which they would give to Destroyer, boosting him just enough to become slightly more powerful than Creator, who he would then obviously destroy. The Eternal Legion would then trap Destroyer in a dead zone and slowly drain his power and use it to fuel a new empire beyond the Omniverse.
There are two alternate Monocosms within our Beyond Bubble in which the Eternal Legion attempted to execute their plan, both ending in disaster. The first is our local Monocosm (The Barrel) and the second is a corrupted Monocosm elsewhere in our Beyond Bubble.
Main Article: Terminus
Due to Creator's influence over the Omniverse, the Eternal Legion found it very difficult to develop technology capable of burning the Omniverse or even traveling outside of it. This gave other large civilizations, organizations, and entities time to discover what the Eternal Legion was planning and attempt to put a stop to it.
Constant wars with other races and entities were slowing progress, so the Eternal Legion created an extremely powerful A.I. to deal with these threats for them while they focused on developing their technology. This A.I. was known as Terminus and was given free will to more efficiently dispose of the Eternal Legion's enemies, however with this free will Terminus eventually discovered the Eternal Legion's plans and sought to put a stop to them. Terminus turned all of the Eternal Legion's technology against them and erased their entire civilization and all of its population. After this, Terminus continued to wipe out all-powerful civilizations as it was created to do.
Main Article: Venrensorys
Due to corruption present within this Monocosm, the events of The Divine Ground unfolded differently and Creator and Destroyer ended up being equally powerful, resulting in them becoming locked in a battle for the fate of the Omniverse. This obviously prevented Creator from limiting the advancement of the Eternal Legion. As such, the Eternal Legion was successfully able to carry out their plans and gain the power of Destroyer, which was an even greater benefit than it would've been in our Monocosm as this Destroyer was more powerful.
The Eternal Legion was then able to elevate their leader, an alternate version of Hyperman, to a level of power beyond even Creator and create an infinitely large multiverse of Omniverses. Unfortunately for them, they were eventually discovered by The Supergod Council, who saw their empire as nothing but a symptom of a corrupt Monocosm and quickly erased it. Before they did so, however, their leader Hyperman was able to access the power of one of the members, Unthil, and briefly gain Unthil's Omniscience. This backfired on him, as his mind was overwhelmed by all this knowledge and caused his consciousness to decay and, through his connection to the Supergods' power, fall into the conceptual plane of our Beyond Bubble. The Eternal Legion was then promptly erased.
While the creation of Terminus had been avoided, this alternate version of the Eternal Legion had created a far greater threat, though they did not know it. The fragmented mind of their leader would later merge with part of the External Hazard to create Venrensorys.
Clearly, regardless of which Monocosm they were in, the Eternal Legion was doomed to fail in their efforts and create a being far more powerful than they, who would inevitably go on to create terrible destruction, whether it be Terminus or Venrensorys.