Army of Darkness
The Army of Darkness has its roots in the final battle of the Grey War. According to The Book of Lore, the forces of light were offered a choice: face the the Darkness immediately, or put it off until some future date. The latter option was selected, and the consequence of this decision was the Breaking of the World.
The army itself is a powerful force of Dolahans backed by Daemons. Unlike the Grey War, the army is controlled by Iname, allowed to choose the time and place at which the army would be released. This was the primary motivation behind the Roads War, and the efforts by the Eladamri to wipe the Iname out.
For a time, it seemed the Ancients had been successful; but the chance survival of one such individual ultimately led to the Army's first attack during the late Sixth Age. The Army would be released a second time during the Age of the Dragon, and finally defeated a few years before the beginning of the Long Night.