Battle of the Maginot Sphere

"Something is wrong! It's moving away! At night I can see it—flitting shadows—black against the stars. Thousands of ships! Not spiraling outward, but heading for the line! This is the tipping point, Didact. It's no longer feeding. It's coming for you."

- The Librarian to the Didact

The Battle of Maginot Sphere was a naval engagement during the Forerunner-Flood war. It marked a turning point in the war, at which point the Flood had stopped feeding and expanding. Instead, the sole devotion of Flood efforts was to penetrating the last Forerunner line of defense, the Maginot Sphere, to complete the destruction of the Forerunners.

Mendicant Bias
The battle was preceded by the rampancy of the Forerunner artificial intelligence Mendicant Bias, who, after a protracted intelligent conversation with the Flood, had decided that the Flood were the next step of evolution for galactic life, and that the Forerunners were corrupt for resisting this inevitable evolution. For refusing to be supplanted by their biologically superior adversary, Mendicant declared his allegiance with the Flood and defamed the Forerunners, not only lending his tactical intelligence to the Flood by directing their fleets, but also gave the thousands of Forerunner-crewed vessels under his command to the Flood. However, his intelligence was tinged with madness, against blind fury for the Forerunners and for their obstinate tendencies, making his actions predictable, and thus easily countered.

Invasion
Shortly after Mendicant Bias assumed command over the Flood forces, he began ordering termination of the Flood's feeding and exponential growth, and instead, Flood forces were marshaled to penetrate the Maginot Sphere, the Forerunner final line of defense for their inner colonies. If the Maginot Sphere was penetrated, the Forerunners would be doomed, thus making this attack an influential battle that would decide the course of the war. Whereas previously captured Forerunner superluminal ships were sequestered and unused in the Flood war effort, Mendicant Bias now recalled all of them to attack the Maginot Sphere, and the gathered Flood attack fleet was over 4.8 million vessels, although the majority were non-military, and an extremely small number were capital-scale warships. The fleet was led by Mendicant's original 1,000 Forerunner warships. The Didact, who was able to predict where the Flood blow would fall upon the sphere, was able to marshal the Suppression, Security, and Emergency Circumstance fleets under the command of Mendicant's "brother", Offensive Bias, with a total of over eleven thousand vessels, with the Flood outnumbering the Forerunners 436.6:1.

Flood assault
Two hours after the Flood fleet appeared, Mendicant Bias made the first move, devoting over a quarter of his vessels — over 1.7 million small and lightweight leisure craft — to storm the Forerunner fleets, not to attack them with weapons, but rather, to overwhelm them, providing too many targets for too few weapons, and boarding Forerunner ships to infect and capture them. Remarkably, the Forerunner vessels were able to evade boarding for three and a half hours. It was after three and a half hours, larger commercial vessels and smaller military craft were launched by the Flood, and the first Forerunner ship was boarded, although it suicidally rammed an incoming Flood ship, releasing over thirty thousand Flood warriors, killing them as the Forerunner crew perished. For four hours, the Flood attack waves intensified, culminating in a final wave of container and transport vessels and warships, most far heavier than a dreadnought.

Firing Halo
"It is best that our crews perished now; because the battle that is about to ensue would have driven them mad."

- Offensive Bias

However, the entire battle up to this point had been a feint — Offensive Bias had merely been playing time and destroying Mendicant's core vessels. As the battle intensified, the Halo Array fired, instantaneously killing all Forerunner and Flood in its area of effect and leaving the majority of the ships lifeless, drifting. The odds were suddenly rebalanced; whereas the Flood had outnumbered the Forerunners initially by millions, now, it was Offensive Bias that had more vessels than Mendicant. In addition, with the crews dead, Offensive Bias no longer needed to concern itself with their safety. It could now accelerate the vessels under its control well beyond velocities safe for biological beings, or open uncontrolled slipspace ruptures across the battlefield, distorting the very fabric of reality around them. Captured Forerunner vessels began self-destruction or opening slipspace rifts that damaged many Flood vessels. The sheer number of drifting derelict ships began to hinder Mendicant's core fleet — they began to collide into the ruptured fuel cells of the drifting vessels, destroying fifty-two of Mendicant's warships in only a few seconds.

Over the next few minutes, over five hundred more of Mendicant's warships were destroyed by collision, the exposed Slipspace rifts, and so on. The battle turned even further — the Forerunner battleships, all controlled by Offensive Bias, now outnumbered Mendicant's fleet six-to-one.

Conclusion
Three minutes later, it was all over. Most of the remaining Flood ships that fled had been reduced to debris, but still one of Mendicant's former Forerunner warships that was part of his original thousand-strong core fleet remained. Offensive Bias retrieved Mendicant Bias's personality construct array and sent it to Installation 00 for research, his mission complete. However, he did not realize that on board one of the Keyships, a fragment of Mendicant Bias remained. This vessel eventually made its way to the San 'Shyuum homeworld and used both in battle during the Sangheili-San 'Shyuum War and later as a power source for their world ship and holy city High Charity.

Trivia

 * The Maginot Line was a French defense, which ran along the Franco-German border to defend against the invading Nazis during World War II. Ultimately, it failed due to being rounded by the German invasion through Belgium and the Netherlands, while a decoy force sat opposite the Line in Germany, deceiving the French into believing that the Germans would attack across that border.
 * The battle itself resembles the Battle of Earth, where Earth was humanity's last stronghold much like the Maginot Sphere was the Forerunner's last line of defense.
 * This battle maybe a homage to the Gainax OVA anime series, Aim for the Top!: Gunbuster, in this six episode long series, mankind faces a race of space insects that act very much like the Flood; they are parasitic, but unlike the Flood they reproduce by infesting stars, causing them to age faster than normal. The final episode has humanity defending a large device capable of creating a black hole in the center of the Galaxy that is designed to kill all space insect creatures living in the center. The alien species, like the Flood, outnumbered their opponents by an unimaginable degree. The Forerunner's too, outnumbered tremendously, stalled for time until the Halo Array activated; much like how the humans held off the space insects until the Black Hole device could be detonated. The human commanders in the series underestimated the intelligence and capabilities of the alien attackers, just how Didact initially showed no surprise to the Flood. During the battle, the space insects attacked in mass waves; hoping to get past the defending Earth ships and physically ram the Black Hole device in an attempt to destroy it, despite their superior technology and firepower, the human ships were often destroyed by the ramming attacks itself; just like how the Flood would use commercial vessels that were many times larger than Forerunner warships to either disable, capture or knock them out.