Halo 3
Just as the player arrives to stop the Prophet of Truth, the Flood infested ship High Charity, arrives in an attempt to take over the Ark. The player, Arbiter and Johnson now take out all the Flood in the area before Arbiter kills the Prophet of Truth and prevents the Halos from being fired. There is no time for the player to rest however, as he must now head to the nuclear reactor at the center of the Flood infested spacecraft, High Charity  in an effort to kill the Flood. Once this is complete, the player and Arbiter must go and activate one Halo ring. This will destroy the Ark and kill all the Flood in this area of the Milky Way Galaxy whilst saving the rest. The player and the Arbiter head off to activate the single Halo and save the Galaxy.