Halo is a created by and now managed and developed by, a subsidiary of . The series centers on an interstellar war between humanity and a theocratic alliance of aliens known as the . The Covenant, led by their religious leaders called the , worship an ancient civilization known as the , who perished in combat with the parasitic . The central focus of the franchise builds off the experiences of , one of a group of codenamed , and his (AI) companion, . Other characters, such as Noble Six from , are also introduced in the series. The term "Halo" refers to the : large, habitable superweapons that were created to destroy the parasitic Flood. They are similar to the in ' novels, and to author 's concept.
The games in the series have been praised as being among the best first-person shooters on a , and are considered the 's "". This led to the term "Halo killer" being used to describe console games that aspire, or are considered, to be better than Halo. Fueled by the success of , and by immense marketing campaigns from publisher , its sequels went on to break various sales records. sold more than 170 million worth of copies in the first twenty-four hours of its release, breaking the record set by three years prior., in turn, broke the record set by Halo 3, selling $200 million worth of copies on its first day of release. Continuing the trend, broke Halo: Reach 's record on its opening day by grossing $220 million. The games have sold over 65 million copies worldwide, with the games alone grossing almost $3.4 billion.
Strong sales of the games led to the franchise's expansion to other media; there are multiple best-selling novels, , and. took the franchise into new territory, as a , while the rest of the games in the series are first-person shooters. Beyond the original trilogy, Bungie developed the expansion , and a prequel, Halo: Reach, their last project for the franchise. A of the first game, entitled , was released on November 15, 2011, exactly ten years after the release of the original. A new installment in a second series of games, Halo 4, was released on November 6, 2012. , a remastered compilation of the four primary Halo titles, was released for the on November 11, 2014.
The cultural impact of the Halo series has been compared by the writer to that of . The collective group of fans of the series is referred to as the "Halo Nation".
In the distant past, a powerful race called the fought an alien parasite known as the . The Flood, which spread through infestation of sentient life, overran much of the . One of the races affected was humanity, who came into conflict with the Forerunners. Exhausted by their war and after having explored all other options, the Forerunners conceived a weapon of last resort to combat the Flood, called the Halo Array. Using an installation known as the Ark, they built large ring-shaped megastructures known as . The Halo Array, when activated, would destroy all sentient life in the galaxy - depriving the Flood of their food, after which samples of the lost lifeforms would be reseeded. Delaying as long as they could, the Forerunners activated the rings and disappeared. Tens of thousands of years later, in the 26th century, humanity - under the auspices of the , or UNSC - colonize many worlds thanks to the development of faster-than-light "" travel. Tensions escalate between the older and more stable "Inner Colonies" and the remote "Outer Colonies", leading to . The UNSC creates an elite group of enhanced supersoldiers, known as, to suppress the rebellion covertly. In the year 2525, human worlds come under attack by a theocratic alliance of alien races known as the . The Covenant leadership declares humanity heretics and an affront to their gods - the Forerunners - and begins a of genocide. The Covenant's superior technology and numbers prove decisive advantages; though effective, the Spartans are too few in number to turn the tide of battle in humanity's favor.
In 2552, the Covenant arrive at the planet Reach, the UNSC's last major stronghold beyond Earth, and launch an invasion of the planet. The last Spartans and UNSC military are unable to stop the Covenant from bombarding the planet. Following directives to avoid leading the Covenant to Earth, , the aboard the fleeing UNSC ship Pillar of Autumn, selects random coordinates that lead the ship to a Halo installation. The Covenant follow, damaging the Autumn and leading its crew to wage a guerrilla war on the ring's surface. The Covenant accidentally release Flood imprisoned on the ring; to nullify the threat, the ring's AI caretaker, , enlists the help of the Spartan , or Master Chief, in activating Halo. Cortana reveals that Halo's activation would mean their own destruction. Master Chief and Cortana instead detonate Autumn 's engines, destroying Halo and preventing the escape of the Flood. They return to Earth with a few other human survivors, warning of an impending Covenant attack. The Covenant, unaware of the destructive nature of the rings, attempt to fire another ring, , to fulfill their religious prophecy and to do so launch an attack on Earth. Helping the Chief to defend the Earth are Sergeant and Commander . One race in the Covenant, the Elites, learn the truth about the rings and are led by a former Elite Commander named the Arbiter to join forces with humanity to stop the firing of the ring. Though they are successful, the unexpected shutdown of the installation triggers a fail-safe protocol, priming all the rings for firing from the Ark. The Covenant High Prophet of Truth and the remaining loyalist Covenant uncover a portal to the Ark on Earth. A battle rages on Earth between human forces, the Covenant, and a Flood-infected army. Eventually, the Covenant is repulsed and flees through a slipspace portal, and the Flood is purged from Earth, resulting in the devastation of much of Africa.
Chief, Arbiter, Elites, Johnson, Keyes, and U.N.S.C. troops follow Truth through the portal, joined by 343 Guilty Spark, who aids the Chief as he has no function to fulfill after the destruction of his ringworld. During the ensuing battle between humans, Covenant, and the Flood on the Ark, Johnson is captured by Truth to activate the Halo rings. Keyes tries to intervene, but is killed. Master Chief and the Arbiter, aided by the Flood, kill the Covenant "prophet" of Truth and rescue Johnson. After finding out that the Ark is constructing a replacement for Guilty Spark's Halo installation, Master Chief decides to fire the unstable Halo, destroying both the Flood and the Ark. After finding out about their plan, Guilty Spark kills Johnson and tries to kill Master Chief, but Master Chief destroys Guilty Spark and activates the Halo. Master Chief and the Arbiter escape on the UNSC frigate Forward Unto Dawn, but the slipspace portal closes with Master Chief's half trapped in space. After a memorial service on Earth for the victims of the Human-Covenant war, the Arbiter and his Elite brethren depart for their home planet. The rear half of Forward Unto Dawn floats in deep space. Cortana activates a distress beacon and the Chief goes into hibernation.
Four years later, the Chief and Cortana awake to find they are being attacked by a Covenant splinter group. Caught in an artificial , they are pulled into the artificial Forerunner planet of Requiem, along with the UNSC ship Infinity. Master Chief, fighting Covenant and AI soldiers called Prometheans, unwittingly releases a hostile Forerunner warrior called the Didact. While seeking a way to combat the Didact, Cortana reveals that she is becoming rampant, a state in AIs when their knowledge base becomes too large and they gradually go insane. Master Chief is eventually contacted by a Forerunner consciousness called the Librarian (The Didact's Wife), who tells him about the Didact: during the crisis with the Flood, the Didact tried to use a device called the Composer to transform humanity into creatures like the Prometheans. He did this because he needed extra soldiers in the war. His own race then imprisoned him along with Forerunner soldiers and humans he had already converted. The Composer was hidden. After the Didact leaves Requiem and retrieves the Composer, Master Chief pursues him to Earth, with Master Chief and Cortana working together to destroy the Composer. Master Chief succeeds, but Cortana sacrifices herself to save him from the resultant explosion. Master Chief is later found floating among the rubble, and taken back to Infinity.
