The Crolun Incident is a name given to a mysterious event which occurred on the Sangheili planet of Crolun in June 2733.
On 26th June 2733, contact was lost with the colony. When a Sangheili exploratory fleet arrived, nothing was found except flaming wreckage, burning fields and levelled cities. The population of the planet were missing- no bodies, nothing. A quarantine was set up by the Sangheili, Hunters and UNSC, and no one set foot on the planet for 400 years after the event.