The Massacre at Eridanus Secundus was a Covenant attack on a rebel facility within a series of asteroids in the Eridanus system in late 2552. Following the Battle of Installation 04 and the rescue of several Spartans and Doctor Catherine Halsey from the surface of Reach after the Fall of Reach, the recovered/captured ship Gettysburg-Ascendant Justice carrying Blue Team, Dr. Halsey, Sergeant Johnson, and several others left Reach, finding themselves nearby the Eridanus system. It was known that a rebel base existed at Eridanus Secundus, so they traveled there, needing major repairs and having no where else to go in the region.
Because of a Forerunner crystal that was recovered on the surface of Reach, the Gettysburg-Ascendant Justice was easily tracked by Covenant forces, and the facility was attacked by a large Covenant fleet not long after their arrival. Though the Gettysburg-Ascendant Justice was able to help defend the facility for a short time, the entire city was evacuated and ultimately destroyed.
These events are depicted in the novel Halo: First Strike.