The Army of Darkness was a massive force of nearly 60,000 yugoloths, ogres, gnolls, goblinoids and several giants. They were led by the Trio Nefarious who organized the monsters into tightly regimented battalions, which made it the biggest army (seeing as a horde, some examples of which rival the Army's numbers, has very little structure) that Faerûn has ever seen. It took twenty-nine months for the army's forces to be gathered.
Each nycaloth had five battalions under their command (except Aulmpiter, who had six). Each battalion was led by a summoned mezzoloth general who each led 5 regiments of troops. These 80 regiments were each led by colonels who were either the chieftains of the larger tribes who now made up the warriors in said regiment or more summoned mezzoloths. Under the colonels were the majors who tended to be the chiefs of the smaller tribes. Under them were the captains and commanders who consisted mainly of the sons and daughters of the aforementioned chieftains, though the giants who joined were all automatically given one of these two ranks (most of them were hill giants who commanded by 'allowing' their mezzoloth second-in-commands to suggest tactics). The commanders each controlled 25 warriors (5 lots of 4 soldiers and a sergeant).
This organization was the key to the success of the Army, without it, Myth Drannor would have been much less likely to fall as it eventually did. The city did only fall however, once the nycaloths were all slain. The monsters reverted to their normal horde tactics and swept down upon the city en masse.
The first military action the army took part in was the attack on the mining and trading encampments in the western Moonsea (eventually this area would become Zhentil Keep) in late autumn of 711 DR. By the Feast of the Moon of that same year, the Army attacked the Cormanthor Forest. This was the beginning of the Weeping War.