Proto-Gravemind

The Proto-Gravemind, unlike other Flood Forms, 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' limbs or weapons and so they do not attack when stationary either.

It is also known as a "Brain-Form", or "Command-Form".

Background
Proto Graveminds are an amalgam of multiple other Flood forms 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.

Proto Graveminds are only seen piloting more complex vehicles that require trained pilots. While Combat Forms can easily pilot smaller vehicles like Warthogs, Scorpion Tanks, Wraiths and Ghosts, Proto Graveminds are needed for larger vehicles like a spaceship. Proto Graveminds have the same color as an Infection Form, but they are bulbous in shape and connect to things by using dark-green appendages.

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 was seen by humans in the Halo: Combat Evolved level, Keyes. This form was made up of several Elite Combat Forms, Human Combat Forms and Captain 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 SpecOps 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.

In the Halo 2 level High Charity, inside the crashed pelican at the start of the level, a proto Gravemind can be seen inside the cockpit, although it can only be seen through the cargo bay.

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 "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 too large. 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. However, many Proto-Graveminds have been seen.

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 and may have been the flood speaking to Master Chief while he was present inside the wreckage of the ship.

trivia

 * You can see Captain Keyes pipe sticking out of the brain form.