Proto-Gravemind

The Brain Form is an uncommon Flood form.

Unlike other Flood forms, 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.

Brain Forms 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.

Brain Forms 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, Brain Forms are needed for larger vehicles like Pelicans and starships.

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

The first Brain form was seen in Halo: Combat Evolved level, Keyes. This form was made up of several Elites, marines, 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 Special Ops forces shortly thereafter.

Another Brain Form 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 ships 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.

Different Brain Forms were also discovered on High Charity during The Battle of Installation 05. These forms were seen piloting Flood controlled Pelican dropships that touched down throughout the city, allowing Combat Forms and Infection Forms to spread quickly and infect as many as possible. Several Forms were also discovered in various parts of the planetoid itself, suggesting that the Flood were also attempting to control High Charity and escape

Further evolution
The Brain Form is thought to further evolve as more bodies are added to it, gathering their intelligences and abilities. The Gravemind, a near-omniscient Flood form made of thousands of bodies is thought to be a highly matured Brain Form. It is comparatively much larger than the other observed Brain Forms, which each contained no more than a dozen bodies. Since there has not yet been an intermediate step observed between a Brain Form and Gravemind, this connection can only be speculated.