Proto-Gravemind

The Proto-Gravemind, also known as the Brain Form, is an advanced Flood form created from the biomass of the most intelligent living organisms available, with an ultimate goal to create a fully-functional Gravemind.

Creation and Physiology
Proto-Graveminds are a coagulation of multiple other Flood forms all merged together. It appears that when an advanced task is at hand for the Flood, such as piloting a starship, they will create a Proto-Gravemind to do that task by merging together as many hosts as possible with that knowledge, such as former pilots and officers. Rather than completely destroying a victim's consciousness, as a normal Flood form generally does, it interrogates its victims slowly, allowing their consciousnesses 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.

A notable feature in the creation of a Proto-Gravemind is that San 'Shyuum are often used to form the core "mind" of the Brain Form, such as Regret, or the Minister of Etiology. The biggest difference between the infected San 'Shyuum and that of the conventional Combat Form, is that examples seen thus far have maintained their personalities to an extent. Other infected forms are subjected completely, to the control of the Infection Form within them and generally display none of the former personality traits of their hosts. On the other hand, infected Prophets have demonstrated the ability to act the same way that they did before they were assimilated. As the Prophet species are considerably weaker and more frail than many other Covenant species, the Prophet would not be a suitable creature to create a Combat Form, and their intelligent nature likely make them more suitable candidates as the main host of a Proto-Gravemind.

Proto-Graveminds are only seen piloting more complex vehicles that require trained pilots. While Combat Forms can easily, albeit clumsily, pilot smaller vehicles like Warthogs, Scorpions, Wraiths, and Ghosts (at least once the Flood infection has reached the Coordinated Stage), Proto-Graveminds are needed for larger vehicles such as a spaceship. Since the Proto-Gravemind does not appear to move at all, it is likely that it directs nearby Combat Forms to pilot the ships, similar to the way a fully-formed Gravemind will direct Flood forms to perform complex tasks or use the Flood Super Cell to pilot the ship.

Unlike all other known Flood forms, it is apparently immobile and passive in combat. They have never been observed to move on their own, even when other Combat Forms are fighting nearby. In fact, even when the Master Chief physically 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 locomotion is only gained after the transformation into a true Gravemind is complete. Proto-Graveminds have also never been observed to keep any of their hosts' weapons. They use some of their hosts' legs as a means to support themselves, and one can slightly see some of the recent hosts' bodies embedded in the Proto-Gravemind's mass, albeit deformed.

Further Evolution
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. Graveminds are created once a Proto-Gravemind reaches a certain "critical mass" and develops a centralized intelligence. This happens only on extremely rare occasions, usually because the Flood is seldom loose on a large enough scale to allow Proto-Graveminds to grow large enough to become a Gravemind. 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 his own. In essence, because it shares all the knowledge of the previous Gravemind and considers them both to be the same being, the Gravemind of the games is the Gravemind that fought the Forerunners, simply been reborn from another Proto-Gravemind.

Appearances
Proto-Graveminds have been observed several times, each time taking on a different appearance to match the different hosts that made up the form.

The first Proto-Gravemind encountered in the Halo Series was in the Halo: Combat Evolved level, Keyes. SPARTAN-117 found this form in the control center of the Truth and Reconciliation. It is possible that the Proto-Gravemind would have allowed the Flood to use the Truth and Reconciliation to spread beyond Alpha Halo. This Proto-Gravemind assimilated Captain Jacob Keyes and attempted to learn from him the coordinates of Earth. Keyes fought with every ounce of willpower he had 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. SPARTAN-117 arrived at the Proto-Gravemind just before 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.

Another Proto-Gravemind was observed on the Covenant ship Infinite Succor at around the same time. This one appeared to be 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. 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 prisoner ship UNSC Mona Lisa, shortly after the destruction of Installation 04. Flood onboard the ship were attempting to access the ship's FTL drives by collecting and absorbing the knowledge of UNSC pilots into this Proto-Gravemind. It was destroyed when Ngoc Benti threw grenades into its interior and blew it up.

A Proto-Gravemind was also present on a Forerunner Shield World, during a mass 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 John Forge and Spartan Red Team.

Trivia

 * If one listens carefully when next to the Proto-Gravemind on the level Keyes, you can hear the Flood form making raspy growling noises.
 * Jacob Keyes is the only notable human to become part of a Proto-Gravemind.
 * In Halo: Combat Evolved, it has also been observed that Plasma Grenades do not stick onto its surface, although sometimes they will land on one of its humps, exploding shortly after, but with no visible effect to the creature. It is unknown why the grenades do not stick on it in the same manner they do to the other Flood forms, although it may be because in Halo: Combat Evolved, the Proto-Gravemind is actually programmed as part of the map.
 * In Halo: Combat Evolved, if one looks carefully, the Proto-Gravemind does move slightly in its place, but not enough to show up on the Motion Tracker.
 * Little bits of liquid will fall off of one of the tentacles sticking from the ceiling.
 * The Proto-Gravemind in Halo: Combat Evolved has seven tentacles.
 * If you examine a Proto-Gravemind carefully, you will see the outlines of different bodies that were used as biomass. This includes Captain Keyes' tortured face.
 * It is known that Keyes' pipe can be found on the Proto-Gravemind in Keyes.
 * When the Proto-Gravemind in Halo Wars is destroyed, it will spurt liquid everywhere. It is possible that Flood biomass functions like a water balloon, having a solid exterior and being filled with liquid. This is why Sniper Rifle rounds have little effect on Combat Forms: the rounds simply pass straight through the liquefied insides, dealing no harm to the Combat Form.
 * Ironically, the Proto-Gravemind was originally called "The Librarian".