Most discussions about the Diablo novels focus on 'The Sin War' trilogy, which essentially acts as the creation myth for the entire game universe. The central plot follows the conflict between the angel Inarius and the demon Lilith, who together fled the eternal war between the High Heavens and the Burning Hells to create the mortal world of Sanctuary. Their rebellious union produced the nephalem, the incredibly powerful ancestors of humanity, which terrified both sides. The story really kicks off with a mortal named Uldyssian ul-Diomed, an unassuming farmer who discovers he possesses latent, world-shaking nephalem powers. As he's hunted by both angelic and demonic forces who want to either control or eradicate humanity, his journey of self-discovery becomes a fight for the very soul of his race.
Uldyssian's awakening draws the attention of the enigmatic being known as the Prophet, who guides him to gather followers and challenge the established religious order, the Triune, which is secretly a front for demonic worship. The plot weaves through his struggles to control his burgeoning abilities, the betrayal of those close to him, and the larger cosmic game where Sanctuary is merely a pawn. The ultimate question the trilogy explores is whether humanity, born of both light and darkness, has the right to determine its own destiny or if it must be purged as an abomination by its paranoid creators. It's less about dungeon crawling and loot and more about the philosophical underpinnings of free will versus preordained cosmic order, all set against a backdrop of medieval-fantasy warfare and clandestine magic.
The trilogy's conclusion sets the stage for the world as we know it in the games, explaining the origins of the Horadrim, the reason angels and demons walk the earth, and the tragic fate of the nephalem's potential. Reading it gives a ton of context to little details in the games, like why Deckard Cain is so important or the true nature of the Worldstone. I always found the moral ambiguity of the angels, who are just as capable of genocidal zeal as the demons they fight, to be one of the most compelling aspects that elevates it beyond simple good versus evil.