Legion is one of the Major Celestials and the Celestial of Chaos and Destruction. He was so destructive that his fellow Celestials imprisoned him in order to protect the galaxy. Out of his chaotic nature he bore a son, Anastar.
Known by many names, the most common of which, in many languages, roughly translates to "Legion", this Celestial is widely regarded as the primordial evil. He is, with the exception of Anastar, generally unrelated to the other, better known Celestials. This is because the way in which he came about was unique, as well as his current absence from the universe.
In the earliest days of the Convergence Universe, when the cosmos was cooling down and order began to spread across it, stars and galaxies were beginning to form. Each galaxy had its own share of star systems and planets, and everything shared a natural order that they followed.
There was, however, one galaxy that was particularly isolated from the rest. While the other galaxies shared a network of clusters and groups, this one could have been considered to be lurking at the very "edge" of the universe itself. In this region, for reasons unknown, the laws of time and space did not behave the same. As a consequence, order was absent and chaos reigned supreme. Stars formed and exploded on a regular basis, planets would come together through the forces of gravity and then come apart for no reason whatsoever. A celestial body would exist in one part of the galaxy, and then suddenly appear somewhere else. The very fabric of space would stretch and tear itself apart, causing cosmic energy to leak through into the normal dimension, and then reform itself and begin the cycle again in an unpredictable order. This resulted in random happenings where things would just simply blink out of existence and sometimes, travel to the past or future and alter the cosmic history of the galaxy altogether.
It was because of this senseless chaos, the utter lack of order and perpetual state of nonsense, combined with the leaking cosmic energy, that the very chaos itself gained sentience.
Shortly after his creation, Legion wasted no time in taking control of the galaxy in which he was born. With his powers, he proceeded to create races that would be naturally adapted to the chaotic habitats of different planets. Millions of races filled the galaxy, each race largely distinct from each other, and each terrifying in appearance and capability. Legion had eventually, over the course of eons, turned his galaxy into a fortress. He then proceeded to use this might to wage war against the Convergence Universe itself. His motivation: to destroy everything in existence and to ensure that chaos reigned supreme throughout the entire universe.
Everywhere Legion touched became afflicted with corruption and chaos. His armies numbered in the trillions and planets with developing, sentient life forms stood no chance against the destruction of their worlds. An unfathomable amount of lives were lost over the course of millions of years. Legion used his powers to warp reality and tear the conquered galaxies apart, before using the raw energy and resources to increase his own fortress galaxy in strength.
As time went on, and Legion became more and more powerful with each attack, the very essence of his corruption infected the fabric of space itself. Civilisations that were lightyears away, who had no knowledge of what was happening would look up at the night sky and unexplainably experience a sudden feeling of dread, fear or unease. Individuals would start making seemingly nonsensical statements such as "the stars have turned against us". Indeed, in some rare cases, locations in the universe that Legion hadn't touched yet would experience irregular space-time anomalies such as stars prematurely detonating and entire planets disappearing and reappearing elsewhere.
Eventually, these events came to the immediate attention of the Celestials. They were at first, confused as they did not know of any entity that was alleged to have as much power as Legion was described. However, when one of them had their first direct encounter with the Celestial of Chaos, the entire Celestial realm decided to take action. Each Celestial rallied their armies - trillions of Eternals, and struck back against Legion and his forces. A small percentage of these Eternals traveled to the furthest reaches of the universe, gathering the aid of several mortal species that had developed into very high levels of technological sophistication. Several millions of these species were gathered together, amongst them being The First, and helped produce an unfathomably large army. The Celestials, the Eternals, and the mortal armies worked together and clashed against Legion and his now equally-large, corrupted forces.
The very battle raged on for millions of years, causing mass amounts of chaos and destruction on a universal scale. Entire galaxies were torn asunder by the conflict, civilisations fell and countless species went extinct. Legion had, by this time, infected enough of space itself to be able to use it against the armies that opposed him, and by gaining access to the Celestial Realm, he wrought a new level of untold havoc on the universe, coming very close to destroying it outright. The combined forces of the other Celestials, however, successfully confronted and banished him from the Celestial Realm and locked him out of it permanently. Now confined to the physical universe, it was easier for the Celestials to fight against him and use their powers in both the Celestial and normal realms to cause heavy casualties and losses against Legion's forces.
While Legion was difficult to fight for any individual Celestial due to the amount of power he had amassed, he was no match for their combined might. Eventually, after the millions of years of grueling war, both sides grew weary from the conflict, though Legion showed these signs sooner and to a larger degree. Slowly, and gradually, his forces wore thin and were eventually either scattered or destroyed. The parts of space that were infected with his corruptive essence were outright erased from existence to prevent its spread. Legion himself was eventually worn down and, after a final climatic battle, defeated. The Celestials trapped him in a physical bod. They bound it in chains and countless enchantments and spells, and then locked the body inside a pocket dimension, which they also enchanted profusely to prevent it from ever being accessible to anyone. After this was done, they proceeded to banish the prison away from the universe itself, leaving it to isolation in the infinite nothingness outside of creation.