On the Sarcophagus, a team of researchers working under command of the Office of Naval Intelligence and alongside Sangheili allies, are confronted with the activation of a Forerunner Guardian at the onset of the Rule of the Created. To disable the Guardian and protect themselves from attack by Cortana, the research facility inhabitants place the world back into a slipspace bubble with the help of the world's Huragoks.
Legacy of Onyx travels from the colony of Paris IV to Earth, then ends on the Sarcophagus.
Legacy of Onyx features a brief appearance by Sergeant Avery Johnson, but focuses mostly on the events in the life of Molly Patel.
Legacy of Onyx takes place during the Raising of the Guardians.