Gravemind

Gravemind is the hive-mind that controls the mysterious alien species known as the Flood. Gravemind resembles a very large venus fly trap. The Gravemind is made up of all the captured minds of the flood victims that have been brought to make it. This is why has not just one mind but many. The Gravemind also has an omniscient ability where it is able to directly control all the flood forms and even speak through them, he is also able to speak telepathically to the Master Chief and The Arbiter.

The Gravemind begins its life as a proto-Gravemind like the one seen in Halo CE. The flood then gather more bodies and minds and incorporate them to form the Gravemind. Strangely the new Gravemind seems to retain all the memories of the previous Gravemind, making it the same being with a new body. The most famous Gravmind is encountered on installation 5 by the Master chief and the Arbiter.

Halo 2
The Gravemind is introduced in the 2004 video game sequel. He is the smartest of the Flood forms. He talks in a slow, low pitched, creepy voice.

He is seen grabbing the Master Chief from Delta Halo's river.

He restrains John, Thel, and Penitent Tangent.

He sends John and Thel to the holy city and the and Installation 05 respectively.

The Gravemind hijacks the In Amber Clad and Stargates it up to the holy city High Charity.

At the end of Halo 2, the Flood and Gravemind have taken over the holy city.

Halo 3
The Gravemind appears voice only in Halo 3.

Floodgate
After UNSC destroy the Covenant anti air guns, the Prophet of Truth flies away in a Forerunner Dreadnought.

The Flood show up in a CCS-class battlecruiser. They crash in the city of Voi.

The Master Chief hunts for the Flood's ship. The Sangheili realize the ship has a message from Cortana, who is trapped in High Charity.

The Covenant
The Gravemind comes in with High Charity and narrowly avoids crashing into the Sangheili's Shadow of Intent.

Later, the Gravemind wants to team up with John and Thel, to stop the Prophet of Truth.

After the prophet is eliminated, the Gravemind turns on the Chief. It keeps John and Thel from escaping, by knocking the two soldiers off the Pelican - which flies away.

John and Thel eventually escape the now-hostile Flood.

Cortana
Master Chief flies to the infested High Charity. Flood infectees attempt to kill him. He eventually finds a trapped Cortana. Chief breaches the dome.

Master Chief detonates High Charity's reactor. He runs from Flood monsters, still trying to kill him.

He finds Vadam fighting the creatures with a UNSC Flamethrower.

The two escape in a damaged, but operable Pelican. The Gravemind attempts to grab the plane. The plane flies off. High Charity goes BOOM!

Halo (Halo 3)
The Gravemind - still not defeated - attempts to rebuild itself on the fresh ringworld.

After the battle with 343GS, the Gravemind is apparently defeated. It dies by getting caught in the explosion.

John and Thel get into Avery's Warthog. The drive through the falling terrain. Flood and Sentinels fight all over the place. The two make it two the UNSC Forward Unto Dawn, and leave the ring.

Forerunner Flood War
The Gravemind was the Flood leader in the Forerunner-Flood War. Presumably, he used Flood Juggernauts. Presumably, he used all the Flood forms from 2531 (Halo Wars).

Quotes

 * "I... I am a monument to all your sins."
 * "There is still time to stop the key from turning. But first it must be found."
 * "Fate had us meet as foes. But this ring will make us brothers."
 * I have beaten fleets of thousands. Consumed a galaxy of flesh and mind and bone.
 * Child of my enemy, why have you come. I offer no forgiveness. A father's sins. Passed to his son.
 * Now. At last I SEE! Her secret is REVEALED!

Halo Dubs

 * You [John-117] are the strongest of the two. But, your potential is wasted on your kind.