A huge plant like thing, made of a collection of Flood. many think that this actually controls the flood, but it isn't a fact. Graveminds form when the flood has been released on a planet (in this case Delta
Halo)for long enough, and is probably the collective minds of everything that is taken by the flood. the "keyes
blob" from the original
halo was probably an early gravemind. He tricked John (Master
Chief for n00bs) and the Arbitor, and took a ship to High Charity. In the end, he asks the rampant Cortanasome questions. Cortana is such a whore.