Gravemind is a mysterious, highly intelligent Flood living inside Halo Installation 05. Little is known about him, other than that he can control the Flood. He appears massive and snake-like, with tentacles. Upon having the Arbiter and Master Chief, he has a conversation with them which initiates the alliance between the Arbiter and Humanity.

Gravemind mostly speaks in iambic heptameter, a type of speech. Gravemind has the Prophet of Truth and the Monitor of Installation 05. Having the Monitor appears to give him the ability to transport all over the Halo, or at least part of him if he needs to get something. At the end of Halo 2, Gravemind is aboard the High Charity speaking to Cortana. Some believe she has either joined him or is being controlled by him, due to her behavior in a trailer for Halo 3. He most likely will have a much bigger role in Halo 3.