The Weeping War was a battle that culminated on 15 Flamerule, 714 DR, in which the Akh'Velahr (the army of Cormanthor) were defeated by the invading demonic forces named the Army of Darkness, leading to the fall of the elven city of Myth Drannor, the greatest and most powerful bastion of civilization by far at the time. https://forgottenrealms.fandom.com/wiki/Weeping_War