Proto-Gravemind

Summary
The brain form is an uncommon Flood type, and only one has ever been observed.

The brain form is a type of Flood unlike all the others because it lacks any sort of mobility. While the infection, combat, and carrier forms are dangerous and always hostile, the brain form is passive in a tactical sense. When the Flood attempt to control a starship or interface with a computer system, the brain form is used.

It normally contains the body of someone who has knowledge of the computer systems necessary for operation. Like the combat form, it can read the thoughts and memories of its host provided he or she is still alive, and it will sift through every memory until it finds what it needs. If it cannot find the information it requires, it will direct other Flood to bring it a new body to search while it consumes the last body.

It interfaces with computers the same way an Infection Form infects a host, through tendrils that attach themselves to the computer's "neural" system. The only time this had ever been witnessed and confirmed was on the Flood-infested Covenant ship Truth and Reconciliation. It is also widely believed that the Gravemind is the evolved product of a brain form.

Flood Brain Forms were also observed piloting Pelicans in High Charity (Level). They also lined the infested halls of High Charity in the same level. They are easily mistaken for large lumps of flesh, but these are indeed Brain Forms.

Speculation
Some believe the term 'Brain Form' may be misleading. The biomasses may have no intelligence at all, and serve merely as a facilitator between combat forms and the technology they must utilize.

Some belive that the "brain form" that consumed Keyes was in fact a mass of many bodies including Elites, Keyes, and probably the Flood Keyes killed before succumbing all mashed into one form and infected so that it could produce more Combat Forms from the bio-mass.

It may be the creature that produces the Infection Forms by breaking down the bio-mass of many creatures and splitting them into small Infection Forms to spread across the area (A bit like a factory or base for the Flood to use when in enemy territory (I.E. a Covenant ship or High Charity).