Although the Covenant often wins over fans as the "best" enemy of the Halo franchise, the Flood is certainly a memorable opponent. Some of the hardest levels in Halo 2 and Halo 3 feature the Flood as the main threat, and they're not an enemy to be taken lightly. Especially the little ones. There is plenty of lore surrounding the Flood that never made its way into the games. With so much out there, most fans are in the dark about various aspects of the terrifying foe. While the Flood does bear a bit of a resemblance to a zombie-like infection, it is far more complicated and intelligent than it may seem. Capable of building networks that can engulf entire planets, the Flood is a force to be reckoned with, and, pulling all the strings, is the Gravemind.
The Gravemind is a character hardly seen in the franchise, but despite the lack of exposure is a presence that is definitely felt. Players will recall battling deep into the Flood infestation to rescue a tortured Cortana from the mastermind behind it all - something that still stands out as one of the series' best all-time moments. Despite going to confront him head on in Halo 3, players only catch a glimpse of the Gravemind in Halo 2 and the details of this being remain a mystery. However, the lore of Halo runs deep, and the Gravemind has been beside it all along the way.
10 In A Long Line of Graveminds...
The Gravemind that players become acquainted with in Halo 2 and Halo 3 is not the only of its kind, and its not the first to have existed. However, the Gravemind itself is an aggregate of all the Graveminds that came before it. It is the main brain of the Flood, directing it wherever and whenever at will, and it possesses the same knowledge of its predecessors, making it more than a simple reincarnation, but rather as a sum of all that the Flood has done and learned over its surprisingly long existence. Just as there were Graveminds to come before it, so there will be Graveminds to come after it. No one was ever really able to keep the Flood down, anyway.
9 As Ancient As The Precursors
The Gravemind has been around for however long the Flood itself has been around, and this is because the Flood, when powerful enough, will form a Gravemind to direct and lead its conquering spread. The Flood has been doing this process for much longer than it seems, stretching back to the civilization that created the Forerunners called the Precursors. The Forerunners rebelled against the Precursors, forcing the remaining Precursors to try and preserve themselves in unusual ways.
Long story short, the process was defective, the ancient Precursors mutated and became the parasitic life forms that would eventually become the Flood, able to absorb other living beings. The first outbreak of the Flood swept across the galaxy, and thus began the war between the Forerunners and the Flood that led to the precious Halo Rings.
8 The Shared Consciousness Of All
In some ways, the Gravemind can be seen as the "leader" of the Flood, but the Flood will create not only one Gravemind, and each Gravemind is not a being of its own. Rather, all Graveminds are the center of the Flood's shared consciousness. Simply put, all the little Flood creatures that players became familiar with are all part of one thinking mind that directs the Flood's strategy and combat through every major outbreak.
As the Flood spreads and infect more beings, the more complex, intelligent, and larger the consciousness becomes, and so, therefore, does the Gravemind. Of all different shapes and sizes and purposes, the Flood never lives or dies, it just waits and grows.
7 Speaks Telepathically
As players became familiar with in Halo 3, the Gravemind can speak telepathically, and does so to the Master Chief. This is a power specific to the Gravemind, as the thought-center of the entire Flood, that isn't really talked about in the games. Telepathy is a strange power that no other creature in the Halo universe seems to have, and that includes the so-called Prophets of the Covenant.
This power, however, isn't unique to the Flood, as it was a power that was widely-used by the Precursors as part of a system they created called "neural physics." Since the Flood were mutated versions of dormant Precursors, it is believed that this made the Flood capable of utilizing the neural physics and speaking telepathically.
6 Absorbs Knowledge Of Taken Hosts
Watching a Flood creature overtake a living being never gets less horrifying. Nor should it. When the Flood infects a new host, it overwhelms and conquers the host's mind. Once inside, it essentially downloads the complete knowledge of that host, then obliterates the rest––individuality, memories, etc. The knowledge that was captured from the host is then transferred to the Gravemind, adding to its vast reservoir of knowledge from other hosts, allowing it to grow smarter, bigger, and stronger.
This is what happens every time a new host is taken by the Flood. Knowing this, watching the Flood decimate hundreds of soldiers only becomes a more frightening prospect, because somewhere the Gravemind is growing stronger with each moment.
5 Can Corrupt AIs
While it may seem as if living beings are the only ones susceptible to the corrupting and devouring powers of the Flood, this simply isn't true. Fans will recall the Gravemind managing to torture Cortana for information, including a particular piece of info that'll be discussed later. This isn't even the half of it, though. During the Forerunner-Flood War, the Forerunners were determined to test the Halo Ring array's effectiveness, and so they sent a special AI system called Mendicant Bias to conduct the test and operate Installation 07.
What occurred instead was a conversation that Mendicant Bias had with an entity known as the Primordial (a Precursor Gravemind), which convinced the AI that the Flood was inevitable. The AI was convinced and switched sides. Even in the face of non-living intelligence, the Flood can be infectious.
4 Skulls For Teeth
As development of the game and the design of the Gravemind progressed, it was said that, at one point, the Gravemind was designed with skulls of infected hosts for teeth. As the Gravemind would be able to speak, so the players would be able to see the empty skulls of Human and Covenant alike, clattering together as they formed horrifying words of fear and death. However, this design was thrown out later on due to the difficulties of attempting to animate the mouth with such intricate parts.
Looking at the visual possibilities of those earlier Halo games, this was probably well-advised. Yet, it's hard not to feel robbed of an image that would've been sure to haunt some nightmares.
3 Engulfs Entire Worlds
In the Flood's attempt to grow indefinitely, the growth of the Graveminds is not subject to an end either, resulting in some cases where the Gravemind was able to engulf an entire planet from fusing organic tissue with the Flood tissue to increase its own size. While the Gravemind never gets to this daunting size in the Halo franchise, it becomes large enough to invade the entirety of the Covenant city called High Charity and devour it, allowing the city to be used as a home base.
Had the Master Chief not been able to stop the Flood in its tracks, perhaps the Gravemind would have overtaken much more and much larger than High Charity.
2 Always Fighting, Always Losing
Since the Flood has been around nearly as long as the Precursors, it also means the Flood has been losing for as long. It fought a war against the Forerunners that pushed the latter to truly desperate measures to rid the galaxy of the Flood, which did succeed in at least pushing the Flood underground. When it reemerged amid the conflict between the Humans and the Covenant, it was poised to take advantage of the chaos and succeed in its aims to dominate the galaxy. However, this too failed (thanks to the Forerunner's desperate measures), and the Flood was defeated again. Should the Flood resurface yet again, it makes one question if they could be defeated again.
1 One Of Only A Few
In addition to the characteristics that define the Gravemind as a unique being in the Halo universe, the Gravemind is special in one piece of knowledge that it knows. It was obtained from the tortured and exhausted Cortana when she was captured. It's a piece of information that only a handful of beings across the entire galaxy know, and a secret that Dr. Halsey has done her utmost to keep hidden from practically anyone and anything. This is Master Chief's real name: John.