Learn everything you need to know about the unstoppable parasites in Halo, known as The Flood, and how they were created.
The Flood are perhaps the most recognizable monster from the Halo series. Alongside The Covenant, The Flood are a series staple that became incredibly popular with the fans and over time their impact on the overarching story only evolved and grew like the shapeshifting creatures themselves.
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First introduced in the original Halo, these body-snatching aliens use the reanimated and mutated husks of humans and Covenant Elites alike to wage a war of terror on the galaxy after the Chief and the crew of the UNSC Pillar Of Autumn accidentally let them loose on the galaxy. And as it turns out, this seemingly unending alien horde has a much larger and deeper background that most people who scratch the surface of Halo in the games will barely see.
The Flood are an absolutely fascinating race and you can lose endless hours combing over every little detail about them throughout the games, animated shows, and countless books. To get you up to speed on these rotting space predators, here’s a general overview of The Flood to provide a refresher for their next appearance.
Referred to in Latin as Inferi Redivivus by Humans, which means the Dead Reincarnated, The Parasite by the Covenant, and the Shaping Sickness by the Forerunners, The Flood have been around for a very long time. First encountered in the Milky Way galaxy 10,000 years before The Forerunner-Flood war that spanned entire galaxies, they’re an alien race that, borne from a parasite, uses the dead to fill their ranks.
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They can infect Humans and Covenant alike and when enough of them mass together they can form a creature known as a Gravemind which uses a hivemind to control lesser Flood forms to carry out monstrous plans and surprisingly efficient tactical assaults and intergalactic campaigns.
Not a lot was known about The Flood's origins at first but in the Halo books Cryptum, Primordium, and through some terminals in Halo 3 it was discovered that they were once an ancient race known as The Precursors. These Lovecraftian monstrosities were creatures of great power that could cross galaxies in seconds and created life on countless planets.
They believed in spreading life wherever they went and that living beings were essential for the galaxy to prosper. They originally created The Forerunners, who eventually grew too prideful and rose up against them, and after a gigantic civil war between the two, The Precursors were seemingly defeated and erased from the universe forever.
However, some of The Precursors had managed to survive by either hiding away and entering hibernation or in most cases, reducing themselves to a cloud of fine dust in order to survive and bide their time. Unfortunately rendering themselves into this state drove most of them insane and those unlucky enough to inhale or accidentally consume this dust would suffer from rampant disease and mutation as the psychotic minds of The Precursors took over and thus over time this ancient race was reborn as The Flood.
Born anew as a shambling unstoppable force of death, fury, and retribution, the galaxy's old owners consumed all biomass and sentient life as they seemingly appeared from nowhere after the galaxy was struck by a mysterious plague that ravaged countless worlds.
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Their vast brutal conquest culminating in a mighty system spanning war with even the mighty advanced technology of The Forerunners unable to stop them until the ultimate sacrifice of wiping out their food source, all sentient life in the galaxy, seemed to at least slow them down and stop them for a few millennia.
That is until some of their surviving spores hidden away on the Halo Ring were accidentally awoken by Chief and his friends, and so they rose once more, furious, insane, and above all, hungry. With one goal in mind, spreading death throughout the entire universe as they carved a bloody path of revenge across the stars.
Before their descent into a shambling mindless monstrosity for The Flood, an infected individual is in most cases fully conscious when the mutation occurs. Though post-death infection is still possible, alive subjects are preferred as there’s less necrosis for the Flood Parasite to deal with.
It’s a pretty unpleasant and painful experience that can drive many to crave the sweet embrace of a bullet not long after exposure. Full mutation into a Combat Form or other more useful body type for the horde occurs relatively quickly so those infected don’t suffer long thankfully.
Although in some rare cases, such as with Captain Keyes and the Prophet of Truth, they were absorbed and used for their information, with the latter becoming an agent for a Gravemind in Halo 3. Either way, it’s not a pleasant experience, and once infected there’s no way to remove or cure the individual other than putting them out of their misery, preferably as painlessly as possible.
Despite their monstrous appearance, The Flood does have a surprisingly efficient hierarchy in place that keeps order and organizes them into an efficient and near unstoppable fighting force. They mutate and evolve into numerous strains and forms to fit roles in the wider structure of their army with each evolution creating more deadly and dangerous creatures to deal with.
From the ground up, you have the basic infectors and carriers which have the sole responsibility of passing the parasite on to as many individuals as possible, dead or alive and screaming in protest. Above them are the combat forms from basic to ranged, stalkers and tanks, the mutated strains varying in efficiency depending on the infected creature.
For example, Marines and Covenant troops still use their original weaponry, whilst larger creatures have extra muscle, limbs, and armored carapaces to charge in, withstand heavy gunfire and give anyone a good whack.
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As they establish a base of operations or a Hive, putrid organic biomass will surround the location and over time it will form a Gravemind. These sentient masses of flesh formed from thousands and thousands of corpses are the penultimate evolved forms of The Flood and they are truly the most dangerous. They use a distributed intelligence formed from all of the other Flood on the planet, creating a hive mind system that lets them control all of their lesser brethren as well as see and know everything within their sphere of influence.
Graveminds are particularly hard to put down as they’re highly intelligent to the point of omniscience and they’re always heavily defended, but without one in control, The Flood quickly descends into a mindless and feral state. This makes them much easier to pick off making Graveminds a highly prized target to any Commander.
Once a planet's biomass is consumed they either form even larger complex structures spanning entire planets known as Key Minds or they turn their ambitions to the stars where they takeover starships of different races and use them in massive parasite-filled armadas to cross the inky blackness of space.
To put it bluntly, you can’t. The Flood are a stubborn enemy to put down and whilst energy weapons, conventional firearms, burning them out with fire and explosions seems to do the job quite efficiently, they can always come back. Similar to the Orks of 40K, The Flood can survive on as small spores that when inhaled start their murderous life cycle anew.
Short of destroying an entire planet once an infection has taken hold, the only successful method of getting rid of them in the past was to basically starve them out. This was managed through isolating infected planets or by using the Halo Ring Weapon System which blasted lethal energy across key points in the galaxy that eradicated all life, Flood and non-Flood alike from nearby planets.
With no biomass left to convert, those that remained after this initial blast simply starved out and died or went inert. Being borderline impossible to get rid of is what makes them such a huge threat, and by the time any incursion on a planet is noticed, it’s already far too late.
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