Diablo

Diablo, whose full name is Al'Diabolos, is the Lord of Terror and the youngest of the three Prime Evils, the ruling demons of the Burning Hells. He is the namesake of the entire Diablo series, and his shadow has hung over every conflict in Sanctuary's history. His brothers are Mephisto and Baal, and he is also known as the Dark Lord, the Prime Evil, Leader of the Three, and Destroyer of Souls. He has a daughter, Leah, born of the witch Adria, and through her a connection to the nephalem lineage. His essence persists even when his body is destroyed, allowing him to return from banishment. Because of this, the forces of Heaven and mortal heroes have repeatedly had to face him anew.

The Prime Evils rarely cooperated, but when they did, Diablo served as the harbinger of their wrath, as at the Fifth Battle of the Diamond Gates. The Eternal Conflict, the endless war between the High Heavens and the Burning Hells, saw him on many fields. On one such battlefield, he fought the archangel Imperius to a standstill, mocking the angel's rage and asking whether Imperius feared how the other angels saw him. Imperius replied that he feared nothing, and then the rest of the Angiris Council, Heaven's ruling council, arrived. When victory seemed close, the brothers turned on each other, a pattern that would repeat throughout their existence.

The Eternal Conflict halted when the Worldstone, a mysterious artifact that maintained the separation between realms, disappeared. Later, human Vizjerei mages summoned beings from the Burning Hells, and Hell learned of Sanctuary, a world inhabited by the offspring of angels and demons. Diablo told his brothers about this new world, and sensing the dormant power within humanity, the three Prime Evils founded the Triune religion to win human belief. Diablo posed as Dialon, the Spirit of Determination. In the last days of the Sin War, the nephalem Uldyssian and his followers, the Edyrem, drew Diablo's attention; his minion Astrogha reported on Uldyssian's might. Even earlier, long before the Dark Exile, Diablo had besieged the great city of Ureh, whose people feared nothing, a display of his obsession with fear.

The Dark Exile occurred when the Lesser Evils rebelled against the three brothers and cast them out of Hell into Sanctuary. The archangel Tyrael assembled the Horadrim, a group of mages, who hunted the Prime Evils and bound each one within a soulstone, a crystal that imprisons a demon's essence. Diablo's stone was buried beneath the cathedral at Tristram. Over two centuries later, King Leoric made Tristram his seat, and Diablo entered the nightmares of Archbishop Lazarus, Leoric's servant, luring him into the labyrinth beneath the cathedral. Through Lazarus, Diablo possessed Leoric's young son, Prince Albrecht. Tristram lost its army and king as demons overran the town. Beneath the ruined monastery, Diablo fed on the fears of the townsfolk and rebuilt his power. Leoric's elder son, Aidan, killed the possessed Albrecht and, in a desperate act, drove Diablo's soulstone into his own forehead to contain the demon.

Diablo's presence overcame Aidan's will. Tormented, Aidan sought solace with the witch Adria, who recognized Diablo's essence within him. Through their liaison, Adria became pregnant, and Diablo intended the child, Leah, to be his future vessel. Aidan, now known as the Dark Wanderer, travelled east to free his brothers. He released Baal from the Tomb of Tal Rasha and then reached Travincal, where he found that Mephisto had already corrupted the Zakarum faith. Diablo used fear to make the uncorrupted defenders flee or turn on each other. The three Prime Evils reunited, and Diablo shed his human host. The heroes of Diablo II killed him in the Chaos Sanctuary, and his soulstone was destroyed. But his essence did not simply vanish; it was banished to the Black Abyss.

In the years that followed, Diablo's return was attempted through various means. Around 1270, the demon Skarn bound Diablo's skull to a throne as a power source to prevent his master from returning. When Skarn was killed, Diablo's soul used the skull to escape the Black Abyss and tried to kill the hero known as the Shard-seeker, but an angel intervened. About two years later, the Cult of Terror used a Worldstone shard to resurrect Albrecht as a monstrous vessel, and Adria, now called the Bride of Hell, explored a plane made from Albrecht's memories. Diablo rebuilt his body, dragging the Shard-seeker into nightmares and sending Terror Demons after them. He mustered an invasion of Sanctuary, the Age of Unmaking, and warped parts of the world into a hellscape, but he was defeated at the World's Crown. Meanwhile, Leah, raised in Caldeum, began showing magic, sometimes in trances as if guided by an unseen hand. Adria had been gathering the essences of the five slain Great Evils into a Black Soulstone, a soulstone powerful enough to hold multiple demonic essences.

By 1285, twenty years after his defeat in Hell, Diablo's servant Adria had captured the essences of Belial and Azmodan as well, completing the Black Soulstone. She betrayed the heroes, and Diablo, merged with all the Prime and Lesser Evils, took Leah's body as his vessel and was reborn as a single Prime Evil. Summoning all of Hell's forces, he approached the Diamond Gates of the High Heavens. The archangel Imperius met him, and Diablo revealed his new true form, cutting Imperius's spear Solarion in two and impaling the archangel. The hero known as the Nephalem defeated Diablo at the Crystal Arch, and his essence was again contained within the Black Soulstone.

While Diablo and the other Evils were contained in the Black Soulstone, it began to corrupt Heaven itself. The archangel Tyrael, reforming the Horadrim, hid the stone at the Tomb of Rakkis, but Malthael, the former Archangel of Wisdom who had become the Angel of Death, took it. Since then, Diablo's essence has persisted. In the era known as the Reign of Enmity, a fragment of Rathma's Prophecy, the Burned Parchment found in the Tormented Ruins, foretells Diablo's inevitable return alongside his brother Baal. Mephisto, appearing as the Bloodied Wolf, confirms that the prophecy speaks true and that his brothers are coming to Sanctuary. Thus, even in the absence of a physical form, the Lord of Terror remains a looming presence, and his return is considered a certainty.