Wikipedia
The Gravemind is a parasitic, hive mind intelligence in the Halo universe. While only one Gravemind is ever seen in the games, the title is given to the final stage of Flood evolution, in which the Flood becomes a superorganism. The Flood is a highly-infectious parasite which is released several times during Halos story. The Chief and the Arbiter (Thel'Vadam, during the course of Halo 2 and Halo 3) are captured during their separate missions on Delta Halo, or Installation 05, by a Gravemind, which resides in the bowels of the ancient Forerunner ringworld, where the Flood creature forges an alliance between the two foes in order to stop the activation of the ringworld — an event which would destroy all sentient life, and, therefore, starve the Flood. The character is voiced by Dee Bradley Baker.
Making its first appearance in Halo 2, the Gravemind was introduced to dispel the idea that the Flood is a mindless virus. The character was designed by a Bungie team, including artists Robert McLees and Juan Ramirez, and slowly developed into a massive creature with tentacles and a frightening level of intelligence. Driven by a desire to spread, the Gravemind is cunning and manipulative; he forges alliances as often as he tries to consume his allies, tricking the Master Chief into aiding him while infecting the Chief's compatriots at the same time. The character has had a mixed reception by many critics upon his appearance in Halo 2, and reviewers including 1UP.com found his role in Halo 3 confusing and without clear motive. More positively, critic Aaron Sagers used Gravemind as an example of a " frenemy" — the creature's appearance made the Master Chief's fight against the Flood more personal and more dramatic.