The First Demonic Crusade was a conflict that took place directly after the 500 Year War. It lasted for several decades and ended in early 2553.
The war notably took place alongside the Human-Triumvirate War.
During its period of recovery, the realm of Hell and its inhabitant demons were without leadership. At numerous times, different demons came forth declaring rulership, only to be challenged by others for the same position. This continuous cycle of bloodshed finally came to an end when the strongest amongst them, a powerful demon named Purloin, successfully rose to the throne. With him he had a huge following which only grew the more his influence spread throughout the realm, until every demon that lived in Hell pledged allegiance to him at last. With his son Endymion by his side as his right hand man, Purloin began to make preparations for a renewed Demonic Crusade against Heaven and Earth.
Humanity had by this time built their own interstellar empire, policed by the United Defense Force (UDF). Scientific advancements continued, and one of the results of these endevours was the ORION Space Marine programme - Humanity's first successful attempt at creating supersoldiers.
This era of peace did not last long however. In 2520, Humanity found themselves at war against an alien faction called "The Triumvirate". Many worlds fell as the UDF rose to halt their aggressive expansion into UEG territory. Unfotunately, most of these attempts were unsuccessful due to the Triumvirate's superior technological and military might. The only effective means of combat the UDF had against them were the Space Marines, though they were too few to really change the tide of the war.
Simultaneously in the midst of the war, news reached the UEG that Heaven was under attack from the revived Demonic Crusade. Already being engaged in one war, the UDF were unable to delegate most of their resources to helping their allies in Heaven. The Naval Intelligence Agency (NIA) of the UEG decided to handpick specific assets belonging to the UDF to aid the Angels against their Demonic adversaries.
One lone Space Marine, Candidate 157 (Aaron) was selected by NIA to work alongside Heaven in their efforts to once again fight against Hell’s forces in a number of tactical operations deep into Hell’s territory. While Humanity continued to wage war against the Triumvirate, in 2524 one lone Space Marine, Candidate 157 (Aaron) was selected by NIA to work alongside Heaven in their efforts to once again fight against Hell’s forces by leading squads of Elite Commandos in a number of tactical operations deep into Hell’s territory. At the same time, the Space Marine was recalled to fight for the UEG against the Triumvirate alongside his fellow Orion candidates.
During his time fighting alongside them, he became well acquainted with the remaining Heroes of Old. The conflict against the Triumvirate lasted until 2552 where Humanity had finally managed to secure a prevailing victory against the Triumvirate for the time being.
By 2553, a final and brutal battle took place between the Heaven and Hell forces which dealt a severe blow to the opposition. At the end of it, Purloin mysteriously disappeared from Hell and his son was forced to take his place. Unlike his father, Endymion struggled with the burden of leadership and desperate for help, he embarked upon a long and grueling journey into some of the forgotten and abandoned lands of the realm. He there, he uncovered a portal of unknown origin and somehow succeeded in activating it, causing him to arrive at a dead world in an uncharted region of space where he discovered a mysterious tomb. Upon entering the tomb, he discovered and accidentally awoke the ancient mummified body of a powerful being that called itself Armageddon.