Battle of Genesis
From Halopedia, the Halo wiki
After seeing a vision from his thought-to-be-deceased AI companion Cortana, Spartan-II John-117 headed to the planet Meridian with the rest of Blue Team to find her. Declared AWOL by the UNSC, Spartan-IV Fireteam Osiris led by Jameson Locke was sent to retrieve them. Osiris tracked Blue Team into an underground Forerunner structure and caught up with them but they managed to escape into a nearby Guardian. Osiris retreated as the Guardian entered slipspace over the planet.
Hunt for Cortana
After arriving on Genesis, a Forerunner Builder installation serving as a link to the Domain, Blue Team fought their way through Jul 'Mdama's Covenant forces who had crashed landed there. They eventually reached a structure where they encountered the Warden Eternal. The Warden introduced himself as Cortana's defender and questioned John-117's motivations while simultaneously deploying several waves of Prometheans to confront the Spartans. After an extended battle, Blue Team were able to defeat the Prometheans, destroying one of the Warden's bodies in the process. Immediately afterward, Blue Team were contacted by Cortana, who informed them of her intention to uphold the Mantle of Responsibility. John voiced concern over the fundamental nature of the Mantle, which were quickly dismissed by Cortana. Fighting their way through several more Promethean squadrons, Blue Team eventually reached the structure known as the Gateway, described as a link between Genesis and the Domain.
Fireteam Osiris arrived on Genesis roughly 48 hours later, having boarded a Guardian near the city of Sunaion on Sanghelios. Upon arriving on the installation, Osiris encountered its monitor, 031 Exuberant Witness, who offered to assist the Spartans in retrieving Blue Team and preventing Cortana from claiming the Mantle. After fighting through the battling forces of Jul 'Mdama's Covenant and Prometheans, Osiris finally reached Blue Team's location, where they encountered two Wardens. After dispatching the Wardens, Osiris headed inside and offered to help Blue Team stop Cortana. In an attempt to prevent John and Blue Team from returning to the UNSC, Cortana transported them to an unidentified location within the interior of the Gateway. Blue Team made their way to Cortana's position after battling through numerous waves of Prometheans and several of the Warden Eternal's bodies. John attempted to persuade Cortana to end her rebellion and return to the UNSC, but Cortana refused and placed Blue Team inside a Cryptum.
Rescue of Blue Team
Learning what had happened, Osiris moved to destroy the gravitational cores holding the Cryptum in place, while fighting Jul 'Mdama's Covenant and Promethean forces. Before Osiris could retrieve Blue Team, however, Cortana removed the Cryptum from its resting point and placed it within a Guardian. In a final effort to free Blue Team, Exuberant led Osiris to an elevator which led to a communications relay from which Cortana maintained control of the Genesis installation. After Osiris defeated several waves of Promethean forces, Exuberant was able to activate the elevator. Osiris made their way to the relay, significantly hampered by the concussive blasts of the Guardian which held the Cryptum. Locke finally reached the relay core and destroyed it, breaking Cortana's control over Genesis and freeing Exuberant to release a swarm of Constructors. The Constructors forced the Cryptum from the Guardian's grasp just before it entered slipspace. The Constructors opened the Cryptum and Blue Team was reunited with Osiris.
Escape from Genesis
Following Cortana’s departure, both Blue Team and Fireteam Osiris together fought their way through hordes of Promethean constructs to reach a Pelican that had previously arrived on the planet via a Guardian’s Slipspace entry. Agreeing to return to Sanghelios, the joint team requested Exuberant’s help, who opened a Slipspace portal for them.
Both teams successfully arrived at Sanghelios and reunited with the Swords of Sanghelios, Doctor Catherine Halsey and Commander Sarah Palmer. The next day, the surviving UNSC forces linked up with the UNSC Infinity. Genesis remained active, now the so-called base for Cortana's Created soldiers.
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