History of Great Disaster

The Great Disaster was the failed Guardian assault on the Hive-held Moon after the Collapse, in which thousands of Guardians attacked Hive fortresses on Mare Imbrium and were routed by the Hive Prince Crota.

Destinypedia places it after the Battle of Burning Lake and before the first game. The Titan Wei Ning fell to Crota; her partner Eriana-3 later led a revenge fireteam against the Hellmouth. The Vanguard ordered a retreat and declared the Moon off-limits except for experienced Guardians.

One Destinypedia account says the failure collapsed the Consensus's authority and led to the three-person fireteam; another says it forced Crota into a long recovery, delaying his invasion of Earth.