Halo 2's Gravemind, holding the Master Chief
|Gender||Host variable, Gravemind N/A|
|Size||Variable, depends on age.|
 The Gravemind
Gravemind is, to all intents an purposes, a Flood. It's simple to say that, but when one looks deeper into it's being, we see that it is not simply like the rest of the mindless infection forms of the Flood that exist solely to occupy, feed and destroy life. One thing that we should state is that the Gravemind is not a unique character. With enough time, food, isolation and safety, a Gravemind can happen anywhere.
A Gravemind is first formed as a Proto-Gravemind. We are not sure how or why certain flood are picked to become Proto-Graveminds, but we can assume that it may have something to do with the host that is chosen. Captain Keyes meets the unfortunate fate of becoming a Proto-Flood in Halo: Combat Evolved. At the time, we did not know what a Proto-Gravemind was, so, with hindsight as an ally, we can assume that maybe the Captain was chosen because he was such an exemplary human. The captain of a start fleet with vast knowledge of Human defenses and weaponry, his knowledge would be a vital tool in learning how to adapt to, and overcome Humans as a race.
A Gravemind can be formed initially from any form of Flood. For whatever reason, after the Flood have infected the host, it is then chosen by the Flood to become a Gravemind and will find a safe place to start it's transformation. Combat forms and pure forms will then feed the Gravemind with Flood infected corpses, presumably so it can absorb knowledge from the human host's brain, to learn and grow. Eventually the Gravemind will end up as a mass of knowledge. Over decades, more and more knowledge absorbed through time will turn the Gravemind into a sort of Ancient being that knows everything about whatever race it has chosen to infect.
 Recorded Gravemind
Captain Keyes met the unfortunate fate of becoming infected by the Flood in Halo: Combat Evolved, aboard the Truth and Reconciliation Covenant Cruiser, after the Flood are released from deep within Installation 04. Later on in the story, Master Chief and Cortana need to go back to rescue the captain so they can use him for authentication to destroy The Pillar of Autumn and destroy Halo. When they get there, however, they find he has been turned into a Proto-Gravemind. If Master Chief hadn't destroyed the Proto-Gravemind's brain to retrieve Captain Keyes' neural implant, it is almost certain that he would've eventually grown into a fully developed Gravemind.
The Gravemind in Halo 2 is only encountered at the start of the level titled "Gravemind". Further to the opening scene, the Gravemind has no contact with the Chief after the intro to the level. Perhaps the level is called Gravemind because it is introducing an enemy that will play a large roll in future events.
In Halo 3, the Gravemind plays a huge part. It is the first time in the Halo Universe that any Flood being is recognised as a character. It has a personality and is apparently a highly advanced Gravemind. It converses with the Master Chief telepathically throughout the entire game.