Halo 2 (part 19) phantom pain

The creature, Gravemind, is the leader of the Flood on Installation 05. The Gravemind reveals to the Arbiter that the Great Journey would destroy the Flood, humans, and Covenant altogether, and sends both the Arbiter and Master Chief to different places to stop Halo's activation. A civil war has broken out among the Covenant; The Flood-infested In Amber Clad crashes into the city, and Cortana realizes that Gravemind used them to take over High Charity.