The Flood’s History In Halo Explained & What They Mean For Season 3

Spoilers are ahead for the Halo season 2 finale.

Although the Halo season 2 finale introduces viewers to the Flood, the episode doesn’t have time to delve into the species’ full history ahead of the show’s third season. In the sophomore outing’s penultimate outing, Kwan Ha (Yerin Ha), Dr. Catherine Halsey (Natascha McElhone), and Commander Miranda Keyes (Olive Gray) band together to solve a cryptic puzzle in the Forerunners’ secret Onyx facility. Relying on the memories of her visions, Kwan solves a star-map riddle that’s stumped the scientists, which opens a door. To the trio’s collective shock, a Forerunner laboratory stands on the other side of the portal.

Dr. Halsey reveals that on a previous stint on Onyx, she found DNA, both “theirs and ours,” in the Forerunner installation. Though surprising, the discovery of the lab tracks. While the Forerunner lab is abandoned, the body of a scientist is curled on the ground, clutching something. Against Kwan’s better judgment, Halsey and Miranda take the Forerunner scientist’s device with them. Insistent on opening the device, Miranda unknowingly unleashes the Flood on the UNSC facility. Since a single Flood spore is capable of destroying an entire species, it’s not surprising that the virulent parasite makes quick work of UNSC personnel.

The Origins Of The Flood In Halo

Why Halo Season 2’s Parasitic Organisms Are Such A Threat

Dubbed “the Parasite” by the Covenant, the Flood is an ancient species that predates both humans and the Covenant by several millennia. The parasitic organisms both reproduce and grow by consuming sentient lifeforms. So long as said lifeforms have enough biomass and intelligence, they’ll feed the Flood’s hunger. A corrupted form of the Precursors — the galaxy’s original holders of the Forerunners’ Mantle of Responsibility — the Flood spreads by infecting lifeforms. Once it does, the Flood hijacks their nervous systems and bodies, causing them to undergo horrifying mutations.

However, the Flood’s physical manifestation isn’t the only chilling part of its nature. In addition to taking on the memories and intelligence of its victims, the Flood forms share one consciousness. The more hosts the Flood bends to its will, the more complex that hive mind becomes. As seen in the cave art that Kwan Ha examines in Halo season 2 “Sanctuary,” the Flood consciousness becomes a monstrous Gravemind entity once it infects enough organisms. Given its hive mind, the Flood is considered a “transsentient superorganism“ — one that is virtually unstoppable if it has biomass to consume (via Halopedia).

The Forerunner-Flood War, Explained

Why The Forerunners Created The Halo Array

The Flood are the reason the Forerunners created not just the Halo Array, but Shield Worlds like Onyx. Centuries and centuries before the Human-Covenant War, a much more brutal conflict unfolded. As the galaxy’s custodians, the Forerunners were poised to fight off the Flood and stop it from consuming everything. After losing legions of their own in the war, the highly advanced beings created several enormous ringworlds (or Halos) as a defensive measure. What the Halo ring does is rather simple: it sterilizes the galaxy, thus starving the Flood by destroying any potential hosts.

Of course, destroying all sentient life in the galaxy is an extreme measure. The Forerunners planned to repopulate the galaxy, hence why their libraries and other installations aren’t just repositories of knowledge, but of genetic information. For this same reason, the Forerunners created Shield Worlds: artificial domains caught inside slipspace bubbles and hidden from the Flood by artificial shells. With Onyx, Halo season 2 introduced a Forerunner Shield World, which is perhaps the only place to hide from not just the Halo Array’s might but the Flood as well.

The Flood’s Role In Halo’s Core Canon

Halo: Combat Evolved Introduces The Species

The first-ever game in the franchise, Halo: Combat Evolved, introduces players not just to the world of Halo but to the Flood as well. After the Pillar of Autumn crashes into Installation 04, the UNSC and the Covenant all stumble upon the Flood. Previously relegated to a containment zone, the Flood infect human and Covenant hosts alike. This prompts 343 Guilty Spark, the Forerunner monitor introduced in Halo season 2’s ending, to enlist Master Chief’s help. Since humans are the new stewards of the galaxy in the Halo timeline, Master Chief is able to activate the Halo Array.

For 343 Guilty Spark, it’s imperative to contain the Flood. The monitor knows just how much damage even a single spore could cause, and is willing to sacrifice all sentient life in Installation 04’s range to ensure the Flood starves. Before Master Chief can activate the Halo, however, Cortana informs him of the weapon’s true purpose. Instead of using the Halo, Master Chief destroys it. In all likelihood, a plot similar to that of Halo: Combat Evolved will unfold in Halo season 3, especially considering the Chief and Makee’s (Charlie Muprhy) differing viewpoints on using the Halo.

What The Flood’s Appearance In The Halo Season 2 Finale Means

The Flood Will Be Halo Season 3’s Main Threat

In the Halo season 2 ending, most deaths are caused by the Flood. From overwhelming Admiral Parangosky (Shabana Azmi) to infecting Dr. Halsey, the organism spreads far and wide in the episode’s short runtime. The way the Flood devastates Onyx is a preview of what could happen in future seasons of the show, and will no doubt factor in to the “blessed ones‘” decision to activate the Halo Array or not. By the end of the episode, Makee makes it clear that she wants to hit a reset button in order to end the war, and the Halo is that.

What sets John apart from other humans in Halo, however, isn’t just his genetic makeup. While that difference certainly allows him to activate Forerunner technology, he has a strong sense of morality that guides how he uses those abilities. For the Chief, protecting humanity at all costs is the mission. It seems unlikely that he’d activate the Halo Array and knowingly sterilize the galaxy, even if that is the best way to stop the Flood. Needless to say, the Flood will serve as future Halo seasons’ primary antagonistic threat.