Leah

Leah was a human woman of mixed heritage, half-demon by blood, who served as the ward of the last Horadrim, Deckard Cain, and called him Uncle Deckard. She was the biological daughter of the witch Adria and Aidan, the prince of Khanduras who had been possessed by Diablo, making the Lord of Terror in a sense her father. Her paternal grandparents were King Leoric and Queen Asylla, while her maternal grandfather was Sevrin. Her uncles included the lost prince Albrecht, and, through Diablo's nature, the other Prime Evils Baal and Mephisto. She was affiliated with the Horadrim, the ancient order dedicated to hunting and binding demons, and with the town of New Tristram. By trade she was a scholar, and her status was that of the deceased, for her life ended when the Prime Evil consumed her.

Her conception was not an accident but a deliberate act of preparation. The three Prime Evils—Diablo, Baal, and Mephisto—had planned their exile to Sanctuary, but Diablo took that plan further. He intended to become a single Prime Evil by combining the essences of all the Great Evils, and for that he needed a vessel. After Prince Aidan struck down Diablo in Tristram and was corrupted by the demon's influence, Adria conceived Leah with him for that specific purpose. Thus, from before her birth, Leah was marked as the container for Diablo's ultimate form, a destiny she did not know until it was too late.

Leah grew up in the city of Caldeum, raised by a woman named Gillian, whose mind had been damaged by contact with demons. Gillian was not always able to function or to care for a child, and the stigma of her madness passed to Leah, whom bullies called sewer rat, partly because she spent much of her time in the sewers to escape. One day she was chased through those sewers by thugs, and that evening Gillian ranted about demons and accused her, a painful memory that stayed with Leah. This difficult childhood shaped her resilience but also left her isolated and uncertain of her origins.

A new guardian appeared when Deckard Cain came to Caldeum and took her in. Crossing the desert with him, Leah reflected that Gillian, for all her madness, had been her mother, and she noted that Cain had called himself Uncle Deckard without explaining how they were related. By a campfire one night, Cain told her that Gillian was not her real mother. They travelled through Lord Brand's sickly town, then to Kurast, where she noticed no animals and greying trees, and onward toward Gea Kul with a monk named Mikulov. During this journey she was abducted and brought before a demon called Rau, who marvelled at her latent power and planned to use it as a lifespark to raise an undead army and summon Belial. When Rau tried to take her blood, she repelled him in a trance with her magic, and Cain rescued her from the Dark Tower.

Over the next thirteen years, Leah and Cain hunted artifacts and lore to piece together the Prophecy of the End of Days, and they studied the Vizjerei, a mage clan known for their demon-summoning practices. Cain tried to teach her to control abilities that he told her came from her mother, though he never revealed the full truth. When the fallen star struck Sanctuary in 1285, Tyrael, the archangel of justice, fell into the Tristram cathedral, and Belial's minions attacked New Tristram. Cain was captured by the Coven, a witch cult, and later killed by the witch Maghda, an event that haunted Leah afterward. She travelled with the Nephalem, the player's hero, and with Tyrael. In Kehjistan she searched Caldeum's sewers and found her mother Adria held captive by the Imperial Guard, and she herself was captured for a time. She tended the Black Soulstone, the stone meant to imprison all the Great Evils, with Adria supervising, and it grew harder to control; she felt everyone expected her to have the answers. Before the siege of Bastion's Keep, she found one of Cain's old journals and read his love for her and his uneasy thoughts about her mother. At Bastion's Keep, Adria stayed at her side during the siege, though that loyalty was a mask.

The betrayal came when Adria revealed her true allegiance. Diablo's essence, with the other Evils contained in the Black Soulstone, took Leah's body as its vessel, and Diablo was reborn as the single Prime Evil. Outside the Diamond Gates of Heaven, Imperius confronted him, and Diablo revealed a true form merged from the seven Evils, partly feminine because of the body he wore. The Nephalem defeated Diablo, but Leah did not survive the ordeal. After her death, Tyrael found her notes at Bastion's Keep and later used them in his own writings. He felt guilt over whether he could have saved her, and when he reformed the Horadrim, he told them about her, fearing that future generations would misunderstand or vilify her. Her story appeared in Diablo III as the main companion, in Reaper of Souls through mentions, in Diablo Immortal as a flashback, and in the novel The Order, as well as in Storm of Light and the Book of Cain.