 Traveling through the portal , the humans and Elites discover an immense artificial structure known as the Ark , far beyond the edges of the Milky Way galaxy . Here , Truth can remotely activate all the Halos . The Flood arrive aboard High Charity in full force , beginning to infest the installation . Truth captures Johnson , as he needs a human to use Forerunner technology . Keyes is killed attempting a rescue , and Johnson is forced to activate the rings . Gravemind forges a truce with the Chief and Arbiter to stop Truth . The Arbiter , Master Chief , and Flood forces arrive and overwhelm Truth 's guards , rescuing Johnson and halting the installations ' activation . After the Arbiter kills Truth , Gravemind turns on the Chief and Arbiter . 
