The New Phoenix Incident was an attack by the Ur-Didact on the Earth city of New Phoenix on July 25th, 2557 using the Composer that he had acquired from Ivanoff Station the day prior. The Ur-Didact, still fostering a hatred for humanity after spending 100,000 years in his cryptum without any outside contact, was attempting to compose all humans on Earth to create his own army of Prometheans. This attack was cut short by the work of John-117 and Cortana, who managed to infiltrate the Didact's ship Mantle's Approach and gain access to the Didact himself. In the ensuing battle, the Didact was restrained by Cortana and knocked into a slipspace rupture. John then used a HAVOK nuclear warhead to stop the Composer and put an end to the attack. While the actions of the Master Chief and Cortana were successful, the entire population of New Phoenix was composed and most of Cortana was destroyed.
These events are depicted in the video game Halo 4.