From Halopedia, the Halo wiki
The Proto-Gravemind, also known as the Brain Form or the Proto-Compound Intelligence, is an advanced Flood form created from the biomass of the most intelligent living organisms available, with the ultimate goal of creating a fully-functional Gravemind. Its creation allows the Flood to move from Feral Stage to Coordinated Stage.
Creation and physiology
Proto-Graveminds are created when the Flood has amassed sufficient bodies and minds to become sentient. In order to create a viable "mind", large numbers of Flood forms and corpses are merged into a globular mass, which uses the combined knowledge derived from each host. They are usually formed in order to handle advanced tasks, such as piloting a starship, in which case Flood forms will merge as many hosts as possible with that knowledge, such as former pilots and officers. The Proto-Gravemind is stationary, and likely directs nearby combat forms to pilot the ships, similar to the way a fully-formed Gravemind will direct Flood forms to execute battle strategies or other complex tasks.
Rather than completely destroying a victim's consciousness, as a normal Flood form generally does, it interrogates its victims slowly, even speaking telepathically to them, and allowing their nervous systems to feed it information. The form tortures the host's mind with a loud buzzing sound, thus erasing all thought. Only after the host is stripped of all knowledge is the host killed by the Proto-Gravemind, although it is sometimes possible for a host to die before the Flood form has a chance to assimilate all of its memories.
Unlike all other known Flood forms, Proto-Graveminds are immobile and passive in combat. They have never been observed to move on their own, even when combat forms are fighting nearby. In fact, even when John-117 punched through the side of the Proto-Gravemind present aboard the Truth and Reconciliation during the Battle of Installation 04, the Flood form did not react at all; it is likely that until the transformation into a true Gravemind is complete, Proto-Graveminds are nothing more than an amalgamation of inactive bodies. In most instances, they use some of their hosts' legs as a means to support themselves, and one can see some of the bodies of the most recently assimilated, embedded in the Proto-Gravemind's mass, albeit deformed. As the corpses become more deformed, a Proto-Gravemind's appearance take on a large body of Flood tissue and biomass.
Proto-Graveminds are capable of jamming or distorting radio signals within a significant radius around them. The means by which they do this is unknown; however, it may be related to the unexplained telepathy the Flood use to communicate with one another. During the Battle of the Etran Harborage, a Proto-Gravemind was shown to be capable of both jamming a real transponder signal and replicating it from its own position to draw in nearby human forces.
The Proto-Gravemind must grow incredibly in size to reach its next stage of evolution, known as a Gravemind, a near-omniscient Flood form made of thousands of host bodies infused with the knowledge of each host that was used to make the Gravemind. While a smaller Proto-Gravemind could direct combat forms, a larger Proto-Gravemind could be in control of multiple Flood colonies for coordinated gestation and attack on nearby life forms. The Proto-Gravemind and Flood colonies also become interdependent on each other; sending colonies into a dormant state would weaken the local Proto-Gravemind, while destroying the Proto-Gravemind would kill the colonies. Graveminds are created once a Proto-Gravemind reaches a certain "critical mass" and develops a centralized intelligence.
Only two Graveminds are known to have ever been created; however, they seem to have the same memories, since the Gravemind present during the Human-Covenant War possessed detailed knowledge concerning the Forerunner-Flood war and even described the actions of the previous Gravemind during that time period as if they were its own.
Proto-Graveminds have been encountered several times, in varying stages of size and potency.
The first recorded Proto-Gravemind was discovered on the surface planet of the Forerunner shield world Etran Harborage during a massive Flood outbreak in 2531. Judging by its huge size and its connection to several Flood colonies, it is likely that this Proto-Gravemind was on the verge of becoming a Gravemind. This Proto-Gravemind was destroyed by UNSC forces led by Sergeant John Forge and Spartan-II Red Team. It had been duplicating Professor Ellen Anders' transponder signal to lure in the UNSC forces and infect them. After the Proto-Gravemind was destroyed, the UNSC Spirit of Fire's forces were able to detect Anders' real signal and follow it into the shield world itself. Its remains were destroyed when the Etran Harborage was destroyed by the Spirit of Fire shortly thereafter.
During the Battle of Installation 04 in 2552, John-117 found a Proto-Gravemind aboard the the bridge of CCS-class battlecruiser Truth and Reconciliation. The Proto-Gravemind would have allowed the Flood to use the Truth and Reconciliation to spread beyond Installation 04. It had assimilated Captain Jacob Keyes and attempted to learn from him the coordinates of Earth. Keyes fought with his willpower to prevent the Flood from gaining this information, burying the knowledge deep down within him and forcing the Proto-Gravemind to systematically discard every other memory he possessed. John-117 arrived at the Proto-Gravemind just as the Flood form was about to finally extract the memory it wanted, and punched through the Captain's skull in order to retrieve his neural implants, thereby robbing the Proto-Gravemind of the knowledge of Earth's location for good. The Proto-Gravemind was destroyed shortly thereafter by the Covenant Special Ops as it was no longer present when human forces captured the ship.
Another Proto-Gravemind was observed on the Covenant ship Infinite Succor during the concurrent Skirmish on Infinite Succor. The biomass was made up of Unggoy, Sangheili, human combat forms, and even animals from the ship's hunting preserve. The form was constructed in the ship's engine room, near the core, probably because the command center of the ship was not accessible to the Flood. Near the end of the battle, the Proto-Gravemind gained the ability to speak and spoke through the infected Minister of Etiology. This form was destroyed by Rtas 'Vadumee before it could pilot the Infinite Succor out of the system.
Yet another Proto-Gravemind was present on the prison ship Mona Lisa shortly after the destruction of Installation 04. Flood on board the ship were attempting to access the ship's FTL drive by collecting and absorbing the knowledge of UNSC pilots into this Proto-Gravemind. It was destroyed during the skirmish onboard when Ngoc Benti killed it with fragmentation grenades.
After Voridus of the Banished released the Flood from High Charity's crash site on Installation 00 in 2559, a Proto-Gravemind quickly formed nearby. One of the tentacles blocked Pavium's retreat and its existence caused the Flood to develop greater coordination and sophistication as time went on, in particular recognizing the threat posed by the controls to reactivate the Ark's Sentinel defense network. This Proto-Gravemind retained a measure of the deceased Gravemind's intellect, but the Gravemind's new vessel had to feed and spread in order for the Gravemind to fully manifest once again. By the time Banished forces discovered the Proto-Gravemind, it was very close to becoming a new Gravemind. Pavium and Voridus led an assault on the creature, aided by the Ark's reactivated Sentinel defense network and Mega Turrets the Banished constructed nearby. After severely damaging the Proto-Gravemind, impairing its impending transformation into a Gravemind with each attack, the Banished were able to clear out a nearby Sentinel lock and summon a Retriever Sentinel. With the help of the added firepower of the Retriever, the Banished were able to kill the Proto-Gravemind. With the Proto-Gravemind dead, the Banished and the Ark's Sentinels were finally able to contain the Flood outbreak.
Halo: Combat Evolved
The Proto-Gravemind first appeared in the Halo: Combat Evolved campaign level Keyes, as a part of the level's Binary Space Partitioning geometry. Although it moves slightly in its place, it does not offer gameplay interactions and cannot be damaged in any way.
The main objective of the Halo Wars campaign level The Flood is destroying the Proto-Gravemind, which is known in-game as a Flood building under the name "Flood Boss". By attacking the Flood colonies throughout the level and sending each into a dormant state, the Proto-Gravemind could be attacked in a weakened state. This may not be necessary, particularly on the lower difficulties. Players could launch a direct attack on the Proto-Gravemind with all of their forces. With upgraded leader powers, particularly the Carpet Bombing and forces mainly made up of Hornets, players could destroy the Proto-Gravemind without much difficulty.
Halo Wars 2
The Proto-Gravemind in the Halo Wars 2: Awakening the Nightmare level Manifestation, which is far bigger and more powerful than the Halo Wars Proto-Gravemind, requires more complicated tactics to defeat. There is also a time limit as the player only has a certain amount of time until the Proto-Gravemind reaches critical mass and becomes a Gravemind. Each time the creature takes damage, the percentage until critical mass drops. First, the player needs to clear out a nearby Flood base and build a Banished Minibase with a Mega Turret. The player must spot the three nearby tentacles so that the Mega Turret can clear them out for the Banished forces to proceed to the creature itself. Before clearing out the tentacles completely, the player should build up their chosen forces to the maximum and upgrade them as far as possible. After reaching the creature, the player must clear out a nearby Flood base and plant a Minibase with a Mega Turret to attack the Proto-Gravemind, causing it to rise up out of the ground, rendering it vulnerable to attack. While fighting off defending Flood forces, the player must target the carapace on the side of the Proto-Gravemind and after blasting through it, blast the exposed sacs. After the sacs are destroyed, the Proto-Gravemind will expose its core which must then be targeted until it retreats and seals itself off. The player must repeat this twice, but on the third time, the Proto-Gravemind will not expose its core as it has become too strong and the Mega Turrets no longer have an effect. On Pavium's orders, the player must clear out a nearby Sentinel lock which will summon a Retriever Sentinel that will then be under the player's control. Dodging Flood attacks, the player must focus all Banished fire, bolstered by the Retriever, upon the Proto-Gravemind until it exposes its core. With the core exposed, target the core until the Proto-Gravemind is killed. This time, the Proto-Gravemind will not retreat when its health is depleted though the player must continue to avoid the Flood forces sent to defend it. Throughout this, the player must build new forces to replace those that they have lost to aid in the attack on the Proto-Gravemind.
List of appearances