The Demonic Legions are the polar opposite of the Heavenly Host. While the Army of Light is orderly and disciplined, the Legion uses brute strength and chaos to overwhelm all who stand in its path.
The Demonic Legions are not comprised of a single type of demon, but all the seven Great Evils of Hell contribute to its power. The exact strength of the Legion is not known but they are innumerable and mostly outnumber their foes rather than use their skills to fight. Each Evil contributes a part of his or her army to the Legion, helping it grow. The Prime Evils, being the strongest of the seven, command the Legion as they please against the Armies of Light.
A hierarchy exists in the Legion: separate divisions and specific operations are often led by trusted lieutenants (such as Shenk or Ghom), and Mallet Lords are overseers that command in the distant areas (where the Seven Evils cannot physically be present at all times).
CompositionEditThe Demonic Legion comprises many different types of demons, big and small. They do not follow much discipline but some ranks like the Oblivion Knights and their underlings, the Doom Knights and Abyss Knights seem to be very disciplined. The Abyss Knights, firing their elemental projectiles in deadly synchronization, while Doom Knights rush forward to hack down those who survived the initial bombardment. If that did not decimate their opponents, the commanders themselves, the Oblivion Knights join in the fray, weakening the enemy with deadly curses and shooting very powerful elemental projectiles that can reduce foes to an almost deathly state.
In spite of the organization of some of the ranks, the majority of the Demonic Legion is very unorganized with blood-thirsty demons vying for destruction, waiting to join in battle.
While each of the Seven Evils has their own army (accordingly named, for example, Army of Destruction for Baal or Army of Sin for Azmodan), any lesser evil may have a group of their followers to fight for them as well. At many occasions, a part of the Legion may rebel against its master: Fallen and Heralds of Pestilence are known examples. Such uprisings are, however, quickly suppressed.
The composition of demonic armies is not constant: it evolves in response for new threats. One such instance is Azmodan creating Demonic Hellflyers to counter the angels' flying troops. Aside from foot soldiers, demons also include monstrous creatures that serve as siege engines: Catapults, Siegebreakers, Demonic Ballistae and Demonic Hell Bearers, to name a few. It is possible that these are products of purposeful breeding as well.
On very special occasions, demons contruct colossal machines of war: the Ram was built to breach the gates of Pandemonium Fortress. The engine was powered by so-called Siege Runes, but those were lost in battle as their keepers were frozen in time within the Timeless Prisons.
The Demonic Legion appears in Diablo III in Act III and in all of Act IV. The Legion greatly outnumbers the forces who dare to drive it back. It is said in the game that Azmodan has led the armies of Hell to many victories against the Heavenly Host, and even almost killed Tyrael.
The Siege of Bastion's KeepEdit
Azmodan, the Lord of Sin led many demons to siege Bastion's Keep to retrieve the Black Soulstone. His army was massive, and appeared to be thousands of demons that poured forth from Mount Arreat. The soldiers of Bastion's Keep managed to hold off the Demonic Legion with Archangel Tyrael's command long enough for the nephalem to kill Azmodan and end the invasion.
Azmodan is said to have been the most talented strategist of the Seven, and the siege of the Keep proved it. He provided a distraction, successfully performed an assault from the depths, broke the defenders' morale, demolished the walls and claimed victory in the battlefields, only losing because of the Nephalem coming to aid.
Fall of the High HeavensEdit
Diablo, the Lord of Terror led the full force of the Demonic Legion into Heaven, and almost managed to destroy the entire Silver City. The angels being completely unprepared for a full scale assault from the demons, the entire Host was scattered and ravaged. If not for the quick actions of the nephalem hero, the Demonic Legion would have destroyed the entire Heavenly Host and all of Heaven with it.
However, the main reason of the assault being so successul was not Diablo's raw might (enough to single-handedly breach The Diamond Gates) and not even the eventual corruption of the Arch. As Tyrael noted, within Diablo (Tathamet reborn) there was no division, and every demon in each of the Realms of Hell obeyed him and him alone. While in the past assaults, the Seven were eventually turning on each other to claim victory for themselves, giving the Angelic Host time to defeat the invaders, the Prime Evil could finish the destruction of the High Heavens, and almost did so, only being defeated due to the Nephalem killing it.
Though all the powers of hell answered to him and him alone, Diablo enlisted various champions to defeat the Angelic Host and to stop the Nephalem. The first to hinder the champion's path is Iskatu, who summons Shadow Vermin and is a expert at vanishing and reappearing. Another is The Lord of Despair Rakanoth, his very presence sends the entire host into a state of hopelessness. The last is Tyrael's former lieutenant Izual the Betrayer, for his ice powers are as cold as his title suggests.
Another unconventional tactic used by the Legion was crippling the angels' leadership beforehand: Imperius was wounded at the start of the assault, Auriel was captured and imprisoned (silencing all hope through out Heaven), Malthael has already left by that moment, Tyrael was in his mortal form and only Itherael was ready to intervene (which he did, directing the Nephalem to their fate).