Gravemind

"I am a monument to all your sins."

- Gravemind Gravemind is one of the main antagonists in the Halo 2 and 3. The Gravemind is a parisitic hive-mind intelligence and is the leader of The Flood.

Forerunner-Flood War
During the Forerunner-Flood War, the first known Gravemind came into existence as a centralized intelligence to coordinate Flood efforts against the Forerunners, formed from the minds of all the Sentients it had consumed and the Precursors which, in their insanity, found it to be a serendipitous emergence and began to guide and feed its growth. It is indicated that the remaining Precursors became the Flood fully, to a lesser or greater extent. Eventually Forerunner space navy tactics began to fail. The Forerunners developed the Halo Array as a desperate countermeasure before the war began fully, which would destroy all sentient life in the Galaxy, thus denying the Flood "food" for growth. This was only to be used as a last resort, but the Forerunners were reluctant to use it; the Master Builder instead abused his powers to have the San'Shyuum Homeworld annihilated.

However, the Flood were growing in numbers, and quickly were gaining ground across the Galaxy. Forerunner commanders realized that their naval tactics were being overcome, and a new solution was necessary if the Forerunners were to pull out of this "stalemate"; a Contender-class Artificial Intelligence, 032 Mendicant Bias, was crafted by the Forerunners to directly attack the Gravemind form and any surrounding Flood biomass in an effort to eliminate the central Flood consciousness. At least four installations, the Ark, Onyx Shield World, Requiem and an unnamed Shield World, were constructed by the Forerunners as shelters for themselves when the Halos fired.

The Gravemind contacted Mendicant Bias and convinced him to abandon the Forerunners and join the Flood cause, convincing him that the Forerunners were so gluttonous and proud as to deny the next step of biological evolution: the Flood. The Gravemind insinuated that by clinging to the legend of the Mantle, the Forerunners had doomed the galaxy to eternal stagnation; the only way for the galaxy to progress was for superior beings to "restart" it. These superior beings, unsurprisingly, took the form of Compound Minds such as Gravemind and Mendicant Bias himself. Convinced by the Gravemind's arguments, Mendicant Bias intentionally became rampant, developed a hatred for his creators and actively worked toward their destruction. The Gravemind utilized a 'Logic-Virus' to subvert other AI's, and may be how he converted Mendicant Bias to his cause originally.

The Gravemind continued to grow, growing enormous, planet-covering nodes of itself called Key-Minds that enhanced its raw computational power to such a degree that it began to completely overwhelm the Forerunners, forcing them into extremely pricey naval engagements that kept the Flood from winning for 300 years. Using knowledge of the Precursors to which it was born from and had become, the Gravemind used the Star Roads of the Precursors to attack the Forerunners, hampering their Slipspace capacity and utterly destroying their fleets, and marking the point at which Forerunner resistance had began to truly break down.

The Forerunners developed a new AI, Offensive Bias, which coordinated the final battle against Mendicant Bias and the Flood. The Gravemind was destroyed or stopped by the first activation of the Halo rings.

Human-Covenant war
Following the Flood outbreak on Installation 05, the Gravemind rebuilt itself deep under the Library of the installation. During the Battle of Installation 05, the Gravemind was capable of utilizing Delta Halo's teleportation grid. After capturingJohn-117 and the Arbiter, the parasite gave them their "assignments" to stop the Halo's activation, and then teleported them to their respective targets. It is not known how the Gravemind, a non-mechanical Flood form, is able to tap into the teleportation grid although it is possible when the Gravemind captured 2401 Penitent Tangent, he absorbed the knowledge of how to access Halo's teleportation grid, just like Cortana on the first Halo when she was in the Control Center.

Gravemind later used the UNSC In Amber Clad to board the Covenant Holy City High Charity, crashing the frigate into a city wall and releasing numerous Pelicans full of the Flood plague. Within days, he took over the entire city, and all life within it.

The Gravemind initially planned to attack and infest Earth, but through the knowledge of the assimilated Regret (and possibly Mercy), he learned about the Prophet of Truth's plans to use the Ark to remotely activate the entire Halo Array. Realizing the extent of the danger he faced, the Gravemind did not attack Earth but instead used High Charity to make a Slipspace jump and crash-land on the megastructure. He briefly allied with John and the Arbiter to stop Truth from activating the rings, but betrayed them the moment the crisis was averted. The duo managed to escape from their foe and discover Cortana's solution to the Flood — a replacement Halo ring made by the Ark. To activate it, John infiltrates High Charity to retrieve Cortana, who has the Activation Index from the first Installation 04.

As John traveled deep into High Charity, he faced multiple Flood forms and heard the Gravemind taunt him, allowing Cortana to briefly send pained messages as he corrupted and tortured the UNSC AI. The Gravemind became increasingly frustrated at the Spartan's progress, becoming suspicious of what Cortana was hiding from him and threatening to "feast upon [John's] bones" if she didn't reveal the secret.

After John rescued Cortana, the Gravemind became enraged, realizing that she planned to use the incomplete Halo to destroy him. He attempted to kill them both, but failed, and John successfully destroyed High Charity, presumably killing the Gravemind. After traveling to Installation 04B, however, John discovered that the Gravemind was attempting to rebuild himself on the new Halo. The Gravemind ultimately failed to stop John from activating the ring and destroying him, at least for the time being.