The driving force behind the parasitic Flood, Gravemind manages to be simultaneously a friend and an enemy of the Master Chief. It and the Chief share a common goal in preventing the Prophet of Truth from firing the Halo array, which would wipe out all sentient life in the galaxy (and therefore kill off the Flood's food source). But this is also a Catch-22 for the human race. Without firing the Halos, the Flood would continue to grow and be a threat. The Gravemind knows this, and it opportunely betrays the Chief once the Prophet of Truth is no more.