Proto-Gravemind

Unlike other Flood forms, the Proto-Gravemind (also known by fans as the Brain Form) is apparently immobile and passive in combat. Brain Forms 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.

Proto-Graveminds are an amalgamate 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 Brain Form 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 and Ghosts, Proto-Graveminds are needed for larger vehicles like starships.

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 brain form of his expertise. The Brain 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.

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 bodies. Graveminds are created once a Brain Form 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 Brain Forms to grow too large. Only two Graveminds are known to have ever been created, however many Proto-Gravemind's have been seen.

Halo 3
Even though the Brain form (Proto-Gravemind) does not appear in Halo 3 it is most probable that there was a Brain form present aboard the Flood Controlled ship that crashed near Voi as the ship would have needed a higher level of intelligence to pilot the complex vessel (like the Brain form in Halo 1)