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All seven of the Great Evils, including Diablo himself, were originally born from the seven heads of a single primordial dragon named Tathamet. Three of those heads became the Prime Evils, and the other four became the so-called Lesser Evils.
Lilith, the demon driving most of Diablo IV's story, isn't some new player in Sanctuary's history. She's the daughter of Mephisto, one of the original Three Prime Evils, which makes the whole conflict feel a lot more personal once you know the family tree.
Mephisto, Baal and Diablo styled themselves as a dark ruling triumvirate over the whole of Hell, using brutal strength and cunning to keep the four Lesser Evils in line beneath them. It's this power imbalance that eventually sparked the rebellion known as the Dark Exile.
Outside of the name players know him by, Diablo also goes by al'Diabolos in older Horadric texts, and followers of the Triune sect refer to him as Dialon, the Spirit of Determination. Different cultures in Sanctuary clearly weren't on the same page about what to call the Lord of Terror.
