Lilith - Mother of Sanctuary
Lilith, the daughter of Mephisto, rebelled against her father and grew dissatisfied with the leadership of Diablo and Baal. She conspired with an angel named Inarius, who shared her desire to escape the Eternal Conflict. Lilith and Inarius stole the Worldstone and created Sanctuary, where they ruled with a council of angels and demons.
They had a son named Rathma, the first of a new race called nephalem. Conflict arose among the renegade angels and demons over the nephalem's power, leading Lilith to massacre her fellow renegades. Inarius banished Lilith to the Void and weakened the nephalem's powers, giving birth to humanity.
Lilith returned during the Sin War and manipulated Uldyssian, a nephalem, to gather followers against the Cathedral of Light and the Temple of the Triune. Eventually, Uldyssian discovered Lilith's true demonic form and their relationship turned hostile. She was once again banished.
After Inarius escaped his confinement in Hell, Lilith once again returned. Her plan was to kill her weakened father and use his power to obliterate both Heaven and Hell and cement herself as the ruler of Sanctuary. She killed her former lover Inarius when he tried to stop her. Lilith was eventually killed by a mysterious wanderer and a group of Horadrim.
The Prime Evils
Mephisto - Lord of Hatred
The oldest of the Three Brothers. Father of Lilith and Lucion. Arguably the most intelligent and cunning demon in existence, though physically the weakest of the three brothers. He was originally captured by the Horadrim in the deserts near the city of Lut Gholein, the same place in which Baal was captured. He was moved to Kurast and imprisoned inside a Soulstone, and was kept locked inside the Guardian Tower beneath the temple city of Travincal. His eventual corruption of the regional religious heads almost ensured his release, except he was eventually defeated by the pursuing adventurer. Being the Lord of Hatred, he is the most intensely evil brother, whose greatest weapon is his victims' irrational hatreds.
Diablo - Lord of Terror
The youngest of the Three Brothers. Father of Leah. He was responsible for the enslavement of mankind with violent images of bloodlust and terror; his prime weapon being the victim's fear. He was originally defeated under the labyrinths beneath Tristram but corrupted the hero who slew him and sought out his brothers, but then was defeated in Hell, by the pursuing adventurer. The aura of the Lord of Terror is the most fearsome among angels and demons alike. As the main antagonist of the series, he is believed to be the most dangerous of them all and has been described by Cain as an "artist of terror" who enjoys sowing fear into people's hearts the way a painter would make brushstrokes.
Baal - Lord of Destruction
The middle child of the Three Brothers. He was captured in the deserts of Aranoch, at the site where his elder brother Mephisto was incarcerated earlier. He was the only one of the Prime Evils to escape the pursuing adventurer. He raised an army of demons and minions and attacked the barbarian homelands of the north, laying siege to the capital city, Sescheron, and finally besieging the last city, Harrogath. He was stopped by the pursuing adventurer and eventually defeated. Being the Lord of Destruction, he is known for his unsparing and annihilating characteristics.
The Lesser Evils
Duriel - Lord of Pain
Duriel, the Lord of Pain, is the "twin" brother of Andariel. Though Duriel had his part in the Dark Exile, he once again aligned with the Three. Sent to guard Tal Rasha's Chamber. His other title is the Maggot King.
Andariel - Maiden of Anguish
Andariel, the Maiden of Anguish, is the "twin" sister of Duriel. Sent to guard the Rogue Monastery by Diablo himself. She is the only female member of the Seven and is the first to fall in the series. Her other title is the Demon Queen.
Belial - Lord of Lies
Belial, the Lord of Lies, is one of the leaders of the rebellion against the Three. He possesses Emperor Hakan II during the Scouring of Caldeum as he attempted to claim the Black Soulstone.
Azmodan - Lord of Sin
Azmodan, the Lord of Sin, is one of the leaders of the rebellion against the Three. Azmodan is said to be a brilliant tactician of the Demonic Army, having almost defeated Archangel Tyrael and the Heavenly Host on many occasions, and considers Belial to be his chief rival. He was also the last Great Evil to be sealed in the Black Soulstone.
Other Demons
Lucion - Firstborn of Mephisto
Lucion was the son of Mephisto and brother of Lilith. He served as the leader of the Triune during the Sin War. He was defeated by Uldyssian and faded into nothing.