With the release of the Diablo 4 trailer, we see that Lilith is summoned through a dark ritual, allowing for her (potential) return to Sanctuary.
In the Lore, it states that following the plan to kill the Nephalem, Lilith was banished to the Void, by Inarius:
According to an ancient text, Lilith was driven into a mad frenzy at the prospect of her children's extinction. She saw the nephalem as the future, and as the means of ending the Eternal Conflict once and for all. She morphed into a new, horrific form, and hunted down her fellow renegades. She ruthlessly murdered each and every follower of Inarius, leaving him to discover the carnage she had wrought. Horrified, Inarius could not bring himself to kill Lilith and banished her from Sanctuary into the Void. He then attuned the Worldstone to cause the powers of the nephalem to diminish over time. Through each subsequent nephalem generation, their powers diminished, effectively giving birth to a weaker, mortal race known as humanity.
And it does state that she did manage to escape the Void once, only to eventually be banished again:
Lilith was twice banished into the Void by Inarius, though escaped the first time.
I assume that the trailer is her being summoned from the Void the second time (though that is only my own speculation at this point), and that may be how she escaped the Void the first time around.
How did Lilith escape the Void the first time?