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."

- Librarian to the Didact

The Battle of Maginot Sphere was a naval battle in the Forerunner-Flood War. It marked a turning point in the war — the Flood had stopped feeding and expanding, and instead, were devoting their efforts 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 and for refusing to be supplanted by their biologically superior adversary. Mendicant declared his allegiance with the Flood and defamed the Forerunners, and not only lended his tactical intelligence to the Flood, directing their fleets, but also gave the thousand Forerunner-crewed vessels under his command to the Flood. However, his intelligence was tinged with madness, against blind fury for the Forerunners for their obstinace, making his actions predictable and thus easily counterable.

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 destined for death, thus making this attack an influential battle that would decide the course of the war. Whereas previously, captured Forerunner supraluminal ships were sequestered and unused in the Flood war effort, now, Mendicant Bias 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 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, with a total of over eleven thousand vessels, with the Flood outnumbering the Forerunners 436.6:1.

Flood attack
Two hours after the Flood fleet appeared, Mendicant Bias had 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 little weapons, and boarding Forerunner ships to infest and capture them. Remarkably, the Forerunner vessels were able to evade boarding for three and a half hours, and despite enemy having too little weapons to target the leisure craft, remained unboarded. 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 Forerunner Dreadnought.

Firing Halo
However, the entire battle up to this point had been a feint — the Didact had merely intended to draw the enemy fleet into range of the Halo Array. As the battle intensified, he fired the rings in a localized pulse, instantaneously killing all Forerunner and Flood in its area of effect and leaving a majority of the ships lifeless, drifting, unpiloted. The odds were suddenly rebalanced; whereas the Flood had outnumbered the Forerunners initially by hundreds and hundreds, now, it was the Forerunners that had more vessels than the Flood. Captured Forerunner vessels began self-destruction or opening uncontained Slipspace rifts that damaged many Flood vessels, and the sheer number of drifting derelict ships began to hinder Mendicant's Flood ships — they began to collide into the ruptured fuel cells of the drifting vessels, destroying fifty-two of Mendicant's warships, and then over five hundred more of Mendicant's warships were destroyed by collision, the exposed Slipspace rifts, and so on. The battle turned even further — with the Forerunners now outnumbering the Flood six-to-one, the Didact began to carelessly maneuver his Dreadnoughts, his aggressive tactics losing some to the Slipspace singularities generated by Flood-captured Forerunner vessels.

Conclusion
Three minutes later, it was all over. Most of the remaining Flood ships fled, but still one of Mendicant's former Forerunner warships that was part of his original thousand-strong core fleet remained. The Didact considered retrieving Mendicant Bias's personality construct array and sending it to Installation 00 for research, and this was eventually carried out, although the Didact mused that Mendicant Bias would have not shown such mercy to him if their positions were reversed. Another implication of the final battle is that with the exception to the Forerunner Dreadnaught, there are a number of Forerunner Ships waiting to discovered by the UNSC or its allies.