Lilith, Mother of Misery, Queen of the Succubi, is a powerful demon, who is of power with the Great Evils (the seven Prime Evils & Lesser Evils combined), but not counted as one. She is daughter of Mephisto, sister of Lucion, mother of Andariel and matriarch of demons.
Background
Lilith is a manipulator and seductress, attractive even in her demonic form. She has always aimed to receive affection from powerful beings, using them as pawns to further her own goals. Such examples include Inarius and Uldyssian.
She was part of the rogue demons and angels who created Sanctuary. Her mating with Inarius resulted in the Nephalem cross between angel and demonkind, a very powerful race of beings who later became the dominant race on Sanctuary, the humans.
However, she desired to use these powerful Nephalem as an army to take on both the Burning Hells as well as the High Heavens and with that came into conflict with Inarius who wanted to undo their creation. Lilith prevented this by killing many of the rogue demons and angels, but ended with her being banished from Sanctuary.
Lilith later returned[1].