SUMMARY
The Battle of Genesis was an engagement between the UNSC, Promethean forces under command of Cortana, and the remains of Jul 'Mdama's Covenant forces on the artificial Forerunner installation known as Genesis in October 2558. While Cortana was attempting to consolidate all the guardians prior to sending them out to control the galaxy, Spartan-II Blue Team and later Fireteam Osiris with help from the facility monitor 031 Exuberant Witness, attempted to stop Cortana. During the conflict, Blue Team was locked away in a Forerunner cryptum, and Osiris was unable to get to Cortana to stop her. Osiris and 031 Exuberant Witness were able to release Blue Team before Cortana could leave with them, however.
RELATED MEDIA
These events are depicted in the video game Halo 5: Guardians, which was the subject of a Spoilercast on November 9th, 2015.