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' 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 significant 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, albeit clumsily, pilot smaller vehicles like Warthogs, Scorpions, Wraiths and Ghosts, Proto-Graveminds are needed for larger vehicles such as 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 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. SPARTAN-117 arrived at the Proto-Gravemind just after Captain Keyes was completely assimilated into the Proto-Gravemind's consciousness. The Proto-Gravemind was destroyed shortly thereafter by the Covenant Special Ops.

Another Proto-Gravemind was observed on the Covenant ship Infinite Succor around the same time. This one appeared to be made up of Unggoys, Sangheili, 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.

Another Proto-Gravemind was present on the prisoner ship UNSC Mona Lisa, shortly after the destruction of Installation 04. Flood on board the ship were attempting to access the ships FTL drives by collecting and absorbing the knowledge of UNSC pilots into this Proto-Gravemind. Presumably it was destroyed when the ship was nuked.

There was also one other Proto-Gravemind at the Flood's main base in Halo Wars. Red Team and a group of marines destroyed the Flood. Because of its size, it is possible that this Proto-Gravemind was close to becoming a Gravemind before its destruction.

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 two games. It is also safe to assume that the Forerunners had to deal with numerous Graveminds.

Halo Wars
In Halo Wars the Proto-Gravemind appears to be the main Flood's base building. The entity controls the many flood colonies from underground and the entire body has a range of miles. The Proto-Gravemind is supported by the several Flood Colonies nearby. It can also be very probable that it was on the verge of becoming a Gravemind, due to its immense size.

This collective was neutralized by Red Team and other UNSC Marine forces.

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.
 * 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 considered part of the map.
 * If you look at it closely, you can see that it does move slightly, possibly to breathe, 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.
 * It has 7 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.
 * Flood Combat Forms most probably make the Proto-Gravemind by dragging bodies to the spot where it is needed, or perhaps they encase themselves in the Proto-Gravemind if bio-mass is at an all time low and its services are imperative to the Flood.
 * 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 liquified insides, dealing no harm to the Combat Form.