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 strike back at the Mevalkagna, and ultimately conquer the entire Omniverse.
There are two notable Monocosms within our Beyond Bubble in which the Eternal Legion attempted to execute their conquest, 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 and Destroyer'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 (most notably The Mevalkagna) were slowing progress, so the Eternal Legion created an extremely powerful A.I. to deal with these threats for them. This A.I. was known as Terminus, but unbeknownst to the rest of the legion, their late High Eternal had downloaded his mind into it in a desperate attempt to cheat death. 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 the poor decisions of two members of The Supergod Council known as Paradoxus and Logixel, this Monocosm was left without any Destroyer to balance Creator, thus causing the Omniverse to overflow with information and civilisations. The Eternal Legion were able to take advantage of this lack of oversight to rapidly increase their technology without limits. Following this, the Eternal Legion were successfully able to carry out their plans and gain the power of Creator.
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 a member of the Supergod Council named Unthil who saw their empire as dangerous and sought to return balance to the Monocosm.
Before they did so, however, their leader Hyperman was able to access the power of Unthil and briefly gain his 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. As Hyperman lost control of the technology linking him to Unthil, it backfired and erased the entire Imperium.
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.