The Lesser Evils are a collective of powerful and influential Demon Lords in the Burning Hells. They are "lesser" only in comparison to the three Prime Evils, who are their only superiors in the hierarchy of the Burning Hells. Compared to every other denizen of Hell, they are ideally supreme.
The Lesser Evils are second only to the Three:
Duriel, the Lord of Pain, is the "twin" brother of Andariel. Though Duriel had his part in the Dark Exile, he has once again aligned with the Three. Sent to guard Tal Rasha's Chamber. Specializes in heavy melee attacks and uses ice as an aura. Has the appearance of a giant bug. He is the second Great Evil to fall in the series and is the only one with no notable backstory other than his participation in the Dark Exile. His aspect is a representation of physical suffering. His other title is the Maggot King.
Andariel, the Maiden of Anguish, is the "twin" sister of Duriel. Sent to guard the Rogue Monastery by Diablo himself. Specializes in mid range poison attacks but ironically enough is weak against the element of fire. She is the only female member of the Seven and is the first to fall in the series. Her aspect is a representation of mental agony. Her other title is the Demon Queen.
Belial, the Lord of Lies, is one of the leaders of the rebellion against the Three. Brother and rival of Azmodan. Uses fire for both phases of his fight with heroes: on the 2nd phase, he'll dig his massive claws into the ground, breathe huge gusts of fel flame and make fiery eruptions. Possesses Emperor Hakan II during the Scouring of Caldeum. His other title is the Vicar of Deception.
Azmodan, the Lord of Sin, is one of the leaders of the rebellion against the Three. Brother and rival of Belial. Throws fire boulders, summons demon gates to call upon minions, rains dead bodies upon enemies, and calls forth dark magic in a large AoE. Azmodan is said to be a brilliant tactician of the Demonic Army, and has almost defeated Archangel Tyrael and the Heavenly Host on many occasions. He is also the last Great Evil to be sealed in the Black Soulstone and is the most intellectually questionable since he deliberately tells the player what his plans are and where to find him, which is the complete opposite of what any reasonable strategist would do in war (thus putting into question exactly how he came close to defeating the forces of Light). His other title the General of Vices.
These are the true names of the lesser of the Great Evils. For ages uncounted each have ruled over their own domains within the Burning Hells, seeking absolute dominion over their infernal brethren. As the Lesser Four continuously vied for the control of those forces that dwelled within their realms, the Greater Three held absolute power over the whole of Hell.
Still, that does not mean that the Lesser Evils are docile servants of the Three. The Lesser Four used dark and evil measures in their quest for power, and at one time managed to overthrow the Greater Three and send them out into the Dark Exile, believing them to have hesitated in the Great Conflict. However, the Lesser Evils were unable to reach a cooperative arrangement as the Greater Three had, and thus, for much of the Dark Exile, the forces of Hell became locked in a civil war, polarized behind either Belial or Azmodan. It is unclear whether Andariel and Duriel took sides in the conflict between Lies and Sin.
In Diablo 2, Deckard Cain theorizes that Andariel's presence in the Rogue Monastery is a sign that the Lesser Evils (at least half of them anyway) are once again united behind the Prime Evils, a theory that seems confirmed with the presence of Duriel in Tal Rasha's Chamber. Their new unity did not last, as the Prime Evils, and most of the Lesser Evils, were slain before Hell could be reorganized.
In Diablo 3, Azmodan and Belial are the last of the Lesser Evils to be defeated, with the latter intending to lead an army to conquer Sanctuary now that he had banished the latter from Hell (thus leaving the Lord of Sin as Hell's sole ruler, with the Lord of Lies as an opposition leader of sorts). However, both demon lords were eventually sucked into the Black Soulstone as per Diablo's plan all along.
| Prime Evil — Tathamet|
Prime Evils — Diablo • Mephisto • Baal
In Diablo III: Reaper of Souls opening cinematic, the Lesser Evils are still trapped in the Black Soulstone along with the Three. When Malthael comes in to take the stone after defeating several Horadrim as well as Tyrael, he shatters the golden bindigs keeping the stone in place and proceeds to grasp it. It is unknown what the reaction is, and will remain that way until said expansion actually comes out.