as a last line of defense against an alien parasite called the Flood, which begins to spread across the ring. Learning that Halo's activation kills all life in the galaxy to prevent the Flood's spread, the Master Chief and Cortana destabilize Pillar of Autumns fusion reactor—destroying it and Halo. Master Chief and Cortana escape in a fighter spacecraft. In the 2003 novel Halo: First Strike, Master Chief and Cortana pick up survivors from Halo and reunite with Halsey and more Spartans. Learning the Covenant has discovered Earth's location, Master Chief leads a preemptive strike against a Covenant fleet. Master Chief returns to Earth in Halo 2 (2004), defending the planet from Covenant attack. Pursuing a fleeing Covenant vessel, Master Chief and the crew of the human ship In Amber Clad discover another Halo ring. Master Chief falls into the clutches of the Flood intelligence known as a Gravemind, who forges an alliance between them and the disgraced Covenant commander known as the Arbiter. The Gravemind sends them to stop Halo's activation—Master Chief is sent to High Charity, a Covenant space station serving as their capital city, then in Halo's orbit. Cortana remains behind on High Charity to ensure the ring is destroyed if activated. At the same time, Master Chief pursues the remaining Covenant leader, the Prophet of Truth, aboard a Forerunner ship. Truth heads to Earth, intent on activating the Halo Array remotely from a place known as the Ark. During the events of Halo 3 (2007), Master Chief reunites with the Arbiter to stop Truth. Master Chief and Arbiter pursue him through a portal to the Ark, located beyond the Milky Way. On the Ark, the Flood-controlled High Charity crashes into the installation. Master Chief stops the Halo Array from firing and battles through the wreckage of High Charity to rescue Cortana. Together, they activate replacement Halo being built on the Arkstopping the Flood while sparing the galaxy at large. Escaping aboard the UNSC ship Forward Unto Dawn, the section of the ship with Master Chief and Cortana is set adrift in space while the Arbiter makes it to Earth. Master Chief does not appear in the games Halo 3: ODST and Halo: Reach, save for a cameo easter egg in Reach. The character returns as the playable protagonist in 2012's Halo 4. After years adrift in space, Cortana awakens Master Chief from cryonic sleep as the Forward