The Raising of the Guardians was a series of events involving the awakening and unearthing of Forerunner ships known as Guardians by Cortana in late October 2558. Following her sacrifice during the New Phoenix Incident, a fragment of Cortana managed to survive, falling into a slipspace bubble and travelling to the Forerunner Builder world known as Genesis. It was there that Cortana discovered the Domain and became convinced that the next inheritors of the Mantle were the “Created”, what she called any artificial minds built by humanity.
By late 2558, Cortana had successfully integrated with the Domain, using it to take control of several Forerunner constructs, most notably the Forerunner Peacemakers known as Guardian Custodes. As she took control of more Guardians, several human, Sangheili, and other colonies were damaged or destroyed, bringing great attention to her and those colonies by the UNSC. Though many forces were brought to bear upon the Guardians, including the UNSC Infinity, Spartan-II Blue Team, and Spartan-IV Fireteam Osiris, their actions were unsuccessful and Cortana was able to take command of the Guardians and control of most, if not all of the populated portions of the galaxy.
These events are depicted in the video game Halo 5: Guardians which was the subject of a spoilercast on November 9th, 2015, the novels Halo: Legacy of Onyx and Halo: Bad Blood, the short stories What Remains and Rossbach’s World from the anthology Halo: Fractures, and the audio drama Hunt the Truth.