Lore in Diablo 4 focuses on the story and setting of the game. Set on the world of Sanctuary and following the story line of the Diablo Franchise.
Diablo 4 Lore
Diablo 4 is set in the world of Sanctuary decades after the events of Diablo III Reaper of Souls.
The world of Sanctuary was originally created by Inarius and Lilith. Inarius was a rogue Archangel that abandoned Heaven after being captured by demons and screamed he wanted to escape from the Eternal Conflict. His captor, Lilith, had the same thoughts about the eternal conflict, so she freed him, and they escaped together. Lilith is the daughter of Mephisto, Lord of Hatred, and shared with Inarius the hatred about the Eternal Conflict. Inarius and Lilith decided to create a new world called Sanctuary away from the Eternal War.
They fell in love, became partners and had mortal children who inhabited the world of Sanctuary called the Nephalem (who are the predecessors of the current population). The Nephalem were stronger than both Angels and Demons, which eventually led them to become a target of both Heaven and Hell for seeing them as a threat. Sanctuary was involved in a war and the Nephalem started dying.
Watching her children die made Lilith mad, so she started fighting both Angels and Demons. Inarius was trubeled from the events and started seeing his children as a menace, and didn't agree with Lilith's actions, so he decided to banish her to the void and attuned the World Stone to diminish the power of the Nephalem after each generation (which resulted in the weak population we can see on the present day).
Diablo 4 Events
Blizzard released a cinematic where Lilith is summoned back to the world of Sanctuary. It's clear that Lilith will play a major role on the events happening on Diablo 4 but remains to be discovered if she will act hostile towards humanity or not as they are the children that she was defending when casted to the void.
When the black soul stone was shattered by Maltheal, the essences reached out into reality. The amount of destruction originated from Maltheal also impacted on the different groups and religions and fights for power started to emerge with no one to stop them because of the lack of leadership and power.