Proto-Gravemind

Very little is known about the Proto-Gravemind, also known as a Brain Form. Unlike other 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. They have never been observed to keep any of their hosts' or weapons. They use some of their hosts legs as feet and you can slightly see some of the hosts' bodies, albeit deformed. They also have not been observed being damaged by small arms fire, although small bits of flesh and Flood matter will come off. It's unknown if this causes the Brain Form any injury.

Background
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. However, only then does the host's life signal die out. It can be argued that the host does not die until that occurs, even though he/she is infected.

Proto-Graveminds are only seen piloting more complex vehicles that require trained pilots. While Combat Forms can easily, aibiet clumsily, pilot smaller vehicles like Warthogs, Scorpion Tanks, Wraiths and Ghosts, Proto-Graveminds are needed for larger vehicles like a spaceship.

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 by humanity was in the Halo: Combat Evolved level, Keyes. This form was made up of several Elite Combat Forms, Human Combat Forms and Captain Jacob Keyes, and was found in the control center of the Truth and Reconciliation, presumably so that it could pilot the vessel and allow the Flood to escape Halo. This form was found by the Master Chief, who killed the newly absorbed Captain Keyes, depriving the form of Keyes' expertise. The form was destroyed by Covenant Special Operational forces shortly thereafter.

Another Proto-Gravemind was observed on the Covenant ship Infinite Succor around the same time. This one appeared to be made up of Grunts, Engineers, Human Combat Forms and 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.

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 Halo 2 Gravemind knew about the history before the Halo array was first activated. This seems to show that each Gravemind inherits the knowledge and memory that the others have obtained. However, many Proto-Graveminds have been seen, although only one in game. Its also safe to assume that the Forerunners had to deal with numerous Graveminds.

Halo 3
Even though a Proto-Gravemind is not present in Halo 3 it is probable that there was one at-hand aboard the Flood Controlled ship that crashed near Voi as the ship would have needed a higher level of intelligence to pilot the complex vessels (like the one in Halo: Combat Evolved). It is possible that the form was not destroyed or killed in the crash, although it is safe to assume it was destroyed when the Elites glassed the area surrounding the Portal.

Halo Wars
In Halo Wars the Proto-Gravemind appears to be the main Flood's base's building

Trivia

 * It is rumored that there is a Proto-Gravemind in the Pelican on the level on High Charity. This is untrue: it is just a hunk of Flood bio-mass.
 * If one listens carefully when next to the Proto-Gravemind on the level Keyes, you can hear the Flood form making raspy growling noises.
 * Keyes is the only notable human to become part of a Proto-Gravemind.
 * 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.
 * If you turn your flashlight off, it will glow a sort of eerie green.
 * If you look at it closely, you can see that it does move slightly, to breathe, but it doesn't show up as a small blip on the motion tracker.
 * Little bits of liquid will fall off of one of the tentacles sticking from the ceiling.
 * It has 7 tentacles, another 7 reference
 * If you examine a Proto-Gravemind carefully, you will see the outlines of different bodies that were used as biomass. This obviously includes Captain Keyes' tortured face.
 * It is known that Keyes' pipe can be found on the creature.