Following the defeat of humanity by the Forerunners, an era of peace followed ... but it didn't last. 10,000 years later, the parasitic Flood re-emerged and began a vicious offensive that brought the Forerunners to their knees.
In the last article, we covered the Precursors, the origins of the Flood, and the Human-Forerunner War, which led to the near extinction of ancient humanity.
Halo timeline: Precursors, Forerunner betrayal, and the era that followed
Now we're going to take a look at the return of the Flood, the fall of the Forerunners, and their ultimate solution against the parasite.
The Forerunners prepare: Two plans to defeat the Flood are conceptualized
The Forerunners spent thousands of years trying to prepare for the Flood.
While most of the Forerunners thought the Flood was nothing more than a human-made hoax to justify invading Forerunner space, many leaders knew the threat was real and began devising ideas on how to prepare for the parasite's return. Two major solutions were conceptualized — Shield Worlds, and Halo arrays.
Shield Worlds were designed to be impenetrable to the Flood.
The Shield Worlds were a concept designed by the Didact (Forerunner Warrior-Servant caste leader) and his wife, the Librarian (the leader of the Forerunners' biological caste, Lifeworkers). They would serve multiple purposes, acting as powerful forward operating bases that could be sent into Flood infested areas and used as a foothold to clear out infestations, while also being safe havens for the galaxy's sentient species, protecting them from Flood infection.
The Halo arrays were proposed by Master Builder Faber, the leader of the Builder caste (the wealthiest, most intelligent members of society). These rings would eliminate all life in the galaxy, including the Flood and potential hosts.
Faber eventually proposed a compromise. Shield Worlds would be used by the Lifeworkers as safe havens for samples of all sentient life, protecting them from the Halo arrays, while the Halos themselves would be used to instantly snuff out all life in the galaxy outside of the Shield Worlds. This plan would allow for the reseeding of the galaxy after the rings fired, but would also ensure the Flood threat was destroyed.
The Didact did not approve of this compromise and stubbornly stood against it. However, the majority of Forerunner leadership felt the idea was their best chance, and as punishment for refusing to accept the terms of the plan, the Didact was sent into exile. Following this, the Builder caste replaced the Warrior-Servants as the Forerunners' military leaders.
The Flood returns: The parasite takes the Forerunners by surprise
Several hundred years after the Didact was exiled, the Flood re-emerged into the galaxy and began to assault the Forerunners. The damage and casualties were catastrophic, but Master Builder Faber chose to try and conventionally defeat the Flood, planning to only use the Halo rings if absolutely necessary.
Success in these efforts was varying. Due to their technological prowess, the Forerunners initially were effectively pushing the Flood back. However, as time went on, the Flood reacquired many of the tools they once held when they were Precursors eons before. These hyper-powerful artifacts outmatched the technology of the Forerunners, and quickly the Flood was able to find ways to disable Forerunner shielding and pierce their armor systems, infecting them easily.
The Forerunners tried dozens of theorized counter-measures against the Flood, but none were significantly successful.
Due to the nature of the Flood, every Forerunner casualty meant reinforcements for the enemy. Despite their vast empire, the Forerunners were quickly finding themselves outnumbered as the Flood continued to infect their soldiers and citizens. Desperate for solutions, the Forerunners created an artificial intelligence construct known as Mendicant Bias. This construct was tasked with bolstering defenses and devising counter-measures to stop the Flood. While it was effective in this, Faber gave Bias a task he deemed extremely important: Questioning the Primordial, the last surviving Precursors, which was now on the seventh Halo ring, Installation 07.
The Primordial and Mendicant Bias engaged in a battle of wits that lasted for 43 years.
By having Mendicant Bias converse with the Primordial, the Forerunners hoped that Bias would find a way to end the Flood threat. However, utilizing the logic plague, a type of computerized virus that takes over the mind of an AI, the Primordial was able to convince Bias to defect to the Flood's side and turn on the Forerunners. Having their own defense AI turn against them was a devastating blow; critical, sensitive intel regarding Forerunner defense plans was revealed to the enemy.
After this, Mendicant Bias utilized Installation 07 to destroy the capital of the Forerunner empire. In a savage and brutal strike, Bias had killed the majority of the Forerunner leadership, with Faber barely managing to escape. This was a crippling blow to the Forerunners. However, the Forerunners were able to recapture Installation 07 and kill the Primordial, who unfortunately was able to transfer his essence into the Flood Gravemind.
The Didact escapes: The Prometheans are created to combat the Flood
At the same time, a young Forerunner by the name of Bornstellar discovered the Didact's exile location. Opening up his containment chamber, Bornstellar freed him from his imprisonment. Unaware of what had happened during his exile, the Didact awoke to a galaxy under siege by the Flood. The pair quickly left, and the Didact imprinted Bornstellar with his conscience and knowledge. During the retreat after the fall of the capital, Master Builder Faber and his forces, alerted to the Didact's escape, ambushed and captured the Didact once more. Wishing to ensure that the Didact would not interfere ever again, Faber imprisoned the Didact in a stasis chamber and stranded him on a ship deep in Flood territory.
He awoke a year later when his stasis chamber failed due to power failure. During his attempts to escape the Flood infestation, he was captured and tortured by the Gravemind, the Flood's central intelligence. After driving him mad, the Gravemind intentionally let the Didact escape, knowing that his return would distract the surviving Forerunner leadership from the Flood.
Upon returning, the Didact discovered that Bornstellar had become a form of a copy of himself known as the IsoDidact — a result of the Didact's imprint that Bornstellar was given prior to the Didact's recapture. The IsoDidact had been given full command of the remaining Forerunner forces, and the Didact (now referred to as the Ur-Didact) chose to retreat to a Shield World called Requiem to begin his own attempts at defeating the Flood.
Utilizing a device known as the Composer, the Ur-Didact killed both Forerunner warriors loyal to him and the majority of the Librarian's samples of ancient humans she had in reserve to seed Earth with. Death by the Composer caused a person to be transformed into a digital essence, and the Ur-Didact placed the essences of those he killed into metal bodies, creating the Prometheans.
The Composer was a devastating machine that was once tried against Flood, to no avail.
While these new soldiers were immune to Flood infection, they were still able to be killed, and they did not slow the tide of the Flood's onslaught. In addition to this, the Ur-Didact's methodology of combating the Flood horrified the Librarian, who traveled to Requiem and (yet again) imprisoned her husband in a containment chamber, afraid to let him freely operate. He would rest dormant there for eons.
All other options are exhausted: The Halo rings are fired
With all other plans failing to work and the Flood closing in on the final Forerunner strongholds, the IsoDidact, Librarian, and Master Builder Faber concluded that it was now time to resort to firing the Halo arrays. Travelling to the Ark, a place where every Halo ring could be fired at once, the IsoDidact initiated the launch, wiping out the Flood and all sentient life in the galaxy, save for the few remaining Forerunners who had found refuge and were now tasked with seeding life back into the galaxy using the stores of species collected by the Lifeworker caste and sheltered in various Shield Worlds.
The Halo rings cleansed the entirety of the galaxy.
With no Flood armies to command, Mendicant Bias was captured and contained in the Ark's systems. The Forerunners, following Mantle of Responsibility protocol, achieved victory and saved the galaxy, though at a monumental cost. Within centuries, the Flood had completely wiped out the Forerunner empire. When their re-population efforts were complete, the surviving Forerunners wished to depart the galaxy in search of a new home, trying to forget the horrors of the past.
Your thoughts
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