Diablo Lore: The Sin War
The Sin War is a defining moment in the history of the Diablo Universe, and if you are going to read any of the numerous stories prior to playing the games, you'll want to start with this trilogy. Inarius, Lilith and Rathma all appear in these stories so they have relevance to the unfolding story in Diablo 4.
The books open with a seemingly innocent story of a farmer, Uldyssian ul-Diomed who is already disillusioned with the current control of the two established religions in Sanctuary. He believes that the Cathedral of Light and the Triune are self-serving and exploit the lower classes for their own means, and his unwillingness to pick a side gets him in hot water and framed for a murder he hasn't committed.
In the course of this tale, he meets and becomes infatuated with a noblewoman who shares his beliefs and wishes to help him raise a rebellion against these two factions, Lylia. Uldyssian begins to manifest powers, which he uses to free himself from captivity and escape with Lylia and a band of rebels. These powers not only grow but he finds himself able to awaken them in others and becomes a spiritual leader of an uprising against The Cathedral of Light and the Triune.
Lylia is revealed to be none other than Lilith, who has managed to escape the Void and return to Sanctuary. She has been manipulating the World Stone under the nose of Inarius to awaken the nephalem powers within humanity, starting with Uldyssian who is later revealed to be a direct descendant of Rathma. Lilith ultimately wants Uldyssian and his followers to join her in an uprising against both Heaven and Hell. Unfortunately, the awakening of this power backfires against Lilith, whose offer is rejected by Uldyssian as he now finds himself strong enough to fight her influence. In fact, Uldyssian can channel the power of the Ancients without her aid and uses this to again banish Lilith to the Void.
These actions don't go unnoticed by the High Heavens who are now aware of Sanctuary, humanity, and the presence of demons in this domain, and the full brunt of the Eternal Conflict is inflicted upon the world. Stopped only by the eventual sacrifice of Uldyssian who gives his life to banish both Angels and Demons from Sanctuary and seal off humanity once again. In this, he also wipes the memories of the humans who were impacted so they can no longer remember the power they once yielded or the existence of Uldyssian himself.
Following this epic battle, both the Angiris Council and the Prime Evils are fully aware of the power that humans are capable of wielding and find themselves in the same quandry that Inarius and Lilith found themselves in at the beginning. The Angiris Council were split on the fate of humanity, with Malthael himself choosing to abstain from the vote. Tyrael himself undecided on humanity's fate, after much thought felt that Uldyssian's sacrifice was proof of the good in humanity, and therefore cast the final vote to leave them to flourish.
The two sides agreed to let the World Stone stay in Sanctuary, call a truce, and agree to sit back and wait. Letting Sanctuary survive as its own domain, and see how humanity advances.
As part of this peace deal, Mephisto demands Inarius as a prisoner. The Angiris Council begrudgingly agree to hand him over, they aren't exactly fond of him themselves after the prior events, and Inarius is then held captive by Mephisto for what we believe is 3000 years of torture.
