Eridanus II was a UNSC colony world in the Eridanus System. The planet was mostly rural, devoid of pollution or crowding, and had climate-controlled weather It was the location of the cities of Luxor and Elysium City, where John-117 grew up until he was abducted and sent into the SPARTAN-II program in 2517. Emile-A239 was from Luxor, born there in 2523.
The Eridanus Rebels had staged a localized rebellion against the UNSC in 2494. The UNSC struck back two years later and forced the rebels out at the cost of four destroyers. A large number of rebels survived, however, and under the command of the UNSC turncoat Colonel Robert Watts, overthrew the civilian government of the system in 2513. Watts was presumed killed in the UNSC's counterattack,Operation: TREBUCHET, but in fact he escaped to Eridanus Secundus in the system's asteroid belt with the rest of the rebels.
As part of Operation: TREBUCHET, Colonial Military Administration Marines attempted to raid an Insurrectionist base on Teribus Island. Sympathizers in the unit tipped the Insurrectionists off, and by the time the dropship arrived it had been abandoned. Later, a nightclub in Elysium City was targeted by Insurrectionist bombers because it was a favorite CMA area for off-duty personnel, killing dozens of civilians and a CMA pilot, Allison Stark and putting a CMA Marine, Eric Santiago, into a coma. Gage Yevgenny and Felicia Sanderson were injured, but survived the attack.
It remained a place of much hostility against the UNSC, as Dr. Catherine Halsey stated many pirates and rebels were operating from the system. John later returned there in 2525 with the other Spartan-II's aboard the Destroyer UNSC Pioneer to board the cargo freighter Laden and travel to a rebel base, Eridanus Secundus in the system's asteroid field. In fact, the asteroid Eridanus Secundus was where John-117 had his first mission, to capture the rebel leader, Col. Robert Watts.
The planet was glassed by the Covenant in 2530 during the Battle of Eridanus II, and the UNSC abandoned the planet.