The Great Disaster was a battle fought between thousands of Guardians and the Hive to take control of the Moon. Shortly after discovering the Hive during the battle of Burning Lake, the Vanguard ordered a full-scale assault on the Moon to establish a beachhead. Lord Shaxx, however, warned the Consensus that the Ascendant Swords carried by the Hive Knights were unlike any weapon he'd seen before, and that they should delay the attack. His pleas were ignored, and the Guardians who arrived fell by the hundreds, as leading the Hive army was the god-prince Crota, an invincible knight who devoured the light of the Guardians. Mare Imbrium was one of the battlefields, as Crota led 1000 knights against over 500 Guardians, easily cutting through them. It was claimed the sky above the region "turned into green fire."
In the aftermath, the Vanguard classified the Moon as forbidden, and not to be visited unless absolutely necessary. One of the survivors of the battle, Eriana-3, swore revenge against Crota and assembled a team of Guardians to enter the Hellmouth alone. Only Eris Morn survived that venture.