Sooo I think I can help out but Im sure Walter can correct me if Im wrong.
According to info so far The Corps defeat and incarcerate most of if not all the Marauders after which Shadow (Hugo Ortiz) starts delving into the Dark Heart nano AI virus which I think they confiscated from the Marauders. He infects himself to upgrade himself but it takes over and he is transformed into Plague and begins his mission to eradicate life or something. lol
There is not a lot of info on the Corps vs Marauders or how the final battle came about
The webcomic showing the 'making' of plague is on the Corps site under wallpaper down the end. If doesn't provide more information, but the imagery maybe useful. The biotags of the current teams are also up there.
There is a bit of solid work on the timeline. The Corps were created after WWII as a Global Response to handle threats, during that period their main adversaries were a group of ex-military terrorists under a single banner called the Marauders: Each time the corps stopped one cell of this group 2 more cells popped up, this conflict lasted throughout the Cold War and in its last incarnation the Marauders consisted of countless foot soldiers and 4 specialists. Ravage, Plague, Vulture, And Roadrash.
During the final war between the Corps! and Marauders The Sinister and Cold Strategist Leader Ravage and His Second in Command Simon "Plague" Crill Used Genetic Alterations and Nano-Machine Bio-science The Marauder Science team coded it as the "Dark-Heart" virus, a complex DNA-Nano Editing virus that enhances Strength to that of a world-class athlete, Halts Aging, and makes the subject completely obedient. The version of the virus Shadow exposed himself to was slightly modified. Under orders from Ravage, the Marauders Allowed the Corps! to know their location knowing the corps! would come.
the Corps team under the command of Terra Team Leader Rucker apprehended the command center and took down the complex mind control devices that commanded the Marauders foot soldiers (a form of hypnotic suggestion and bio-engineered nano-tech that made their foot soldiers amazingly resilient in the battlefield).
Simon Crill was the last of the Marauders to be captured, and even though he was captured he swore that the Marauders had already won, and events had been set into motion to assure the Corps! were already doomed.....
The final Marauders battle was a smokescreen to Ravage's ultimate plan to destroy the corps from within, he knew only Covert command agent Shadow would be able to breech their defenses and set up a series of attacks to wound Shadow, baiting him every step on the way. And the plan worked! Shadow was wounded, the Marauders were captured, but that was a short-lived victory,Shadow against orders used the confiscated Nano-Machine tech that the marauders were using to try to repair his broken body, it worked......But there was a Trojan horse within the Nano-machines....
The "Dark Heart" Nano virus, this code rewrote and bio-hacked shadow, he became the namesake of the creator of the nano-machines... Shadow's personality and memories were rewritten into a version of the architect of the virus, PLAGUE! And the war with the Marauders was over, and thus began the war between the CORPS! and it's once friend, these events were too much for the Corps! leader Rucker, he stepped down and relinquished command to Rain.
Thus begun the Battle of a new Enemy, one designed by the Marauders but able to replicate itself a thousand fold! THE CURSE Army!
Within the virus was replication tech that allowed cloning of damaged tissue, ***It's important to note this as this is where The Curse Acquired the fallen Corps! Marksman Darius Weaver who was handpicked by the newly appointed leader of the corps Rain.
Darius was a loyal corps operative until a fateful raid on a Curse compound that changed Darius forever. During the raid one of Smoke's bombs detonated near Darius, disfiguring his face from the resulting shrapnel. The Corps evacuated the compound before it collapsed and left Darius behind thinking he was surely killed in the blast. Little did they know, he had actually been saved by an enemy Curse member! left behind Plague used the "Dark Heart" virus on Darius and begun his quest of the ultimate ground assault force, giving Darius an new persona SHRAPNEL, although most of his memory has been wiped clean, he still harbors resentment towards Smoke for the bomb accident that scarred his entire body. He has since been enhanced with nanotechnology by the Curse to create the perfect foot soldier for the Replication Program-
The Replication Program Clones hosts into perfect "Replicas" and every Replication is loyal to Plague!