Cortana wakes Master Chief from cryonic sleep shortly before forces from a newly formed Covenant splinter faction board the vessel. The remnants of Forward Unto Dawn are caught in Requiem's gravity well and pass through an opening to crash-land on its interior. As Chief and Cortana explore Requiem, fighting hostile forces from the new Covenant faction and mechanical warriors called "Promethean Knights", Cortana malfunctions. She reveals that she is experiencing "rampancy", a declining mental state faced by smart AIs past their usual seven-year life span. Chief promises to get Cortana to Earth, believing that Cortana's creator, Doctor Catherine Halsey, can fix Cortana's condition. Chief and Cortana pick up garbled transmissions from a human ship, UNSC Infinity, who have picked up the Dawn's distress call. Cortana attempts to warn Infinity away from Requiem's gravity well and directs the Chief to deactivate what she believes are communications jammers. Instead, the Chief unwittingly releases the Didact, an ancient Forerunner warrior, from imprisonment. The Didact, who deems Man unworthy of the Mantle of Responsibility, takes control of the Prometheans and the Covenant faction, then attacks Infinity after it is dragged into Requiem.
The player's head-up display (HUD) shows real-time information on the player character's armor system, such as shield status, information on current weapons and abilities, and waypoints for goals and objectives. The HUD also has a motion tracker that detects allies, enemies, and vehicles within a certain radius of the player. The game sees the return of the alien species who were formerly members of the Covenant as foes, and introduces a new type of enemy called the Prometheans, which are Forerunner artificial intelligence (AI) constructs. There are three types of Prometheans: Knights serve as leaders of the group and are considered the deadliest of the Promethean forces; Crawlers are a weaker class that often attack in packs; and Watchers offer support and have the ability to shield or revive Promethean allies.
Source: Wikipedia, "Halo 4", available under the CC-BY-SA License.
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