Lilith is the main protagonist in Diablo 4 and features in the main Diablo 4 cinematic.
Diablo 4 Visual Appearance
Lilith's visual appearance is somewhat more refined than we have seen in Diablo 2 which was simply another version of her daughter Andariel. Her striking stature and wings stood out in the Diablo 4 cinematic reveal.
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.
Sanctuary and Nephalem
Lilith 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, Lilith 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.
The Sin War
Lilith later returned and tried once more to use the Nephalem against her perceived opponents, weakening the Worldstone (that prevents Nephalem from developing their powers) and helped awaken Uldyssian's hidden Nephalem powers. She tried to seduce him into creating the army of Nephalem she wanted, but ultimately failed as Uldyssian used his power against her.
Inarius also helped stopping her from behind the scenes, as was Rathma, and he banished her once more from Sanctuary.
It has been debated if this part of Lilith's story is canon or not, since it's a Battle.net only event.
As the heroes searched for Diablo, and killed legions of Succubi and Lilith's daughter Andariel, she decided to intervene personally.
She masterminded the revival of Diablo by manifesting herself in the mortal realm and performing a ritual of Dark Summoning. This ritual, long thought to be impossible by the most powerful mortal wizards, allowed Lilith to link with Diablo across realms, strengthening him until he was finally able to once again reincarnate himself in the mortal plane.
Rage and a deep yearning for revenge boiled within the eyes of the Lord of Terror, and together he and Lilith resurrected the two remaining Prime Evils as well as Duriel and Izual, two of their strongest agents.
Diablo announced his grand plan - the Three were finally strong enough to spread Pandemonium throughout Sanctuary. Lilith could have easily conducted an assault on her own, but because she had been left severely weakened by the Dark Summoning ritual, she agreed to aid the Prime Evils and command some of their remaining forces.
Diablo 4 and Lilith Timeline Explained
Blizzard explains the Diablo 4 timeline and how Lilith returns as follows:
Diablo 4 takes place decades after the events of Reaper of Souls. Though Malthael’s plans were foiled, the Angel of Death exacted a terrible toll on humanity and Sanctuary suffered grave devastation. Entire populations decimated, cities ravaged, heaven itself suffered great losses causing them to shutter their gates.
You can imagine what that does to people. Religious schisms, famine, war, basically humanity left to our own devices and we know how well we do on our own.
It is against this backdrop of despair that Lilith returns and that’s when the events in the cinematic happened. This is important because in the mythos of Sanctuary Lilith is the mother of humanity. Lilith represents that demonic half that we all have inside our hearts. In that mythos she and the angle Inarius along with other rebel angles and rebel demons tired of this endless great conflict created a world that would be free of conflict called Sanctuary and along with that created the first mortals. That’s the legend, that’s what we’ve heard.
The scary part of that is that she knows our hearts, she knows what motivates us and can manipulate us. The other scary part of that is she is Mephisto’s daughter and she represents a link to an even great threat, the Greater Evils.
Even though she was summoned for reasons we are not ready to divulge just yet we know it can’t be good. With Heaven having closed their gates there’s no one to answer our prayers. Evan the Horadrim are but mad old men talking to themselves, talking to graves, clutching their bottles are hard as they once clutched their swords.
Lilith in Previous Diablo Games
Lilith appeared in Diablo 2 and was also known as Uber Andariel. She is one of the Uber boss monsters you encounter during the Pandemonium Event.
She's could be found in the Matron's Den and drops Diablo's Horn. Here appearance in Diablo 2 was as a larger version of Andariel, with higher damage, more hit points, and a few other bonuses.
Lilith's Diablo 2 Appearance
Lilith's true form is that of a tall, humanoid Demoness. Her striking appearance includes four long, jointed limbs extend from her back and reach in front of her, two on each side, each with a large spike at the end. Her hands each have four fingers ending in claws. She has large breasts and a feminine face with a mouth full of fangs, and hooves rather than human feet.