Similar to other games in the series, much of Diablo 4's lore is communicated through in-game texts and compendiums, but a physical lore and art book released alongside the game has now shed some light into the fates of the three Prime Evils during the timeline in which Diablo 4 takes place. Set roughly 30 years after the events of Diablo 3, the latest game in the series puts players into the realm of Sanctuary where Mephisto's daughter Lilith has returned. Lilith's goal is to empower the denizens of Sanctuary to fight back against Hell itself (albeit at the cost of enslavement and servitude to Lilith) while also eliminating the Horadrim.
The Horadrim play a critical role in Diablo 4 and the timeline of the entire franchise, as they are the order created by the archangel Tyrael to combat the three Prime Evils -- Baal, Mephisto, and Diablo. Lore for Diablo 4 reveals that the Horadrim are in possession of the remnants of each of the Prime Evils, including the Horn of Diablo. However, in order to better understand why the Horadrim would want these items and why Lilith wants to destroy them, it's necessary to look back thousands of years prior to the events of the original Diablo.
The Endless War, Sanctuary's Creation, and Diablo's Return
According to the lore of the Diablo series (which has been told across countless in-game texts as well as via novels, comics, art books, and fan analysis), the timeline begins with the existence of a mighty, unfathomably powerful spirit -- Anu. Once Anu becomes self-aware, it immediately casts all evil from itself to form the multi-headed beast Tathamet. The two entities engage in a conflict that lasts countless millennia until finally destroying each other with a critical blow. The site where both beings die is Pandemonium and Tathamet's death results in the creation of Hell and all demons, including the Prime Evils -- Baal, Mephisto, and Diablo.
Of course, Anu's death results in the creation of the High Heavens and Angels, and thus the spawn of Anu and Tathamet are embroiled in a second Endless War, waging for thousands of years with no end in sight. Seeing the futility of the conflict, the angel Inarius gathers sympathetic angels and demons and partners with Mephisto's daughter Lilith to steal Anu's eye, the Worldstone, and create the realm of Sanctuary that will be free of the conflict. Lilith and Inarius' children, the Nephalem, inspire other angels and demons to coexist peacefully and give birth to the race known as the Ancients, from which humanity will spawn. Roughly 3000 years later, humans who carry darkness in their hearts heed the call of Diablo and help to aid in his resurrection.
What Happened to Diablo After the Events of Diablo 3
Diablo's resurrection at the hands of King Leoric and his servant Archbishop Lazarus kick-starts the entire Diablo franchise timeline and brings players to the concurrent storyline spread across 4 mainline entries and the mobile spinoff Diablo Immortal. In each of the games, players ultimately face off against the titular Prime Evil Diablo as the final boss and main antagonist. This culminates in the demon's appearance in Diablo 3, where players traverse through Pandemonium itself and ultimately arrive at the Crystal Arch of Heaven to banish Diablo once and for all.
In the intervening years between the ending of Diablo 3 and the events of Diablo 4, the new order of Horadrim obtain Diablo's remains. Along with the Eye of Baal and the Brain of Mephisto, the Horadrim are now in possession of the Horn of Diablo. Each of these objects are intended for the Horadrim to research as part of their mission to understand and combat against the demons of Hell. Since the Horadrim stand between Lilith and her goal of enslaving humanity to unlock their hidden potential as Ancients, she seeks to destroy both the order and any remnants of the Prime Evils.
Diablo 4 is available now for PC, PS4, PS5, Xbox One, and Xbox Series X/S.