She is tertiary antagonist of Reaper of Souls.
Very little is known from Adria's past, mostly due to her usually avoiding the subject. She was the daughter of a merchant, who neglected her, and after he died when their house was caught in a fire she vanished. Wandering in the wilderness, Adria was quickly labeled a witch by those she encountered. She eventually joined the Coven, an order of witches, along with Maghda, and the two eventually became the leaders of the order. Years later, Adria decided to leave the order, around the same time as the Darkening of Tristam.
Adria moved to Tristam after hearing of the events transpiring there, building a hut in the outskirts of the town seemingly overnight. There, she offered her help to the adventurers who sought to fight the evil coming from the monastery in the edge of town, seeing this as an opportunity to get some gold and magical artifacts. Many of the villagers feared her, as she made no secret of being a witch. Deckard Cain was the only one who approached her, as the two exchanged their knowledge on arcane texts.
All adventurers who came to Tristam would either flee or die in the depths of the cathedral, but one day three adventurers returned, having defeated the source of the evil there: none other than Diablo, the Lord of Terror. One of them was Aidan, son of King Leoric, who was visibly traumatized by the experience, so he sought solace with Adria.
Between Diablo and Diablo III
When Adria looked at Aidan, she saw the image of Diablo on him. As it turned out, Aidan had plunged Diablo's soulstone into his head, in an attempt to contain Diablo's essence within him. The two spent many nights alone, and when Aidan left Tristam (now wearing the guise of the Dark Wanderer), Adria talked with Cain, telling him that Aidan left to the west. She also left the village shortly afterwards, along with the barmaid Gillian.
The two traveled to Caldeum, where Adria gave birth to Leah. She left Leah in Gillian's care and departed. During his travels in pursuit of the Dark Wanderer, Deckard Cain briefly met with Adria, which is when he learned of Leah's existence. After giving a scroll to Cain, Adria once again vanished.
Seven years after the destruction of the Worldstone at Mt. Arreat, Cain received a report on Adria's death. However, he doubted it, as he continued to sense Adria's presence nearby, though he was unable to locate her. He eventually learned that Adria passed through the Great Library of Caldeum, seeking information on the Desolate Sands and the ruins of the Cathedral of Light. Cain then realized she had been searching for the Black Soulstone.
During their travel to Caldeum, which had been sieged by Belial, the archangel Tyrael, the Nephalem and Leah find Adria in the city sewers, cornered by Belial's minion. After being rescued by them, Adria tells she planned to use the Black Soulstone to contain the essence of all the Great Evils and destroy them. This whole time she had successfully tracked the essences of Diablo, Baal, Mephisto, Andariel and Duriel, and thus only Belial and Azmodan were left. However, in order to find the Black Soulstone, she needed to revive Zoltun Kulle, the creator of the artifact. So, while Tyrael and the Nephalem set off after Zoltun Kulle and the Black Soulstone, Leah stayed with her mother, who taught her to control her magical powers.
After The group kills Azmodan on Bastion's Keep, Adria told they'd have to return to the keep, as to make the preparations to destroy the stone. However, once they get there Adria kills all the guards and incapacitates Tyrael, the Nephalem and Leah. She then reveals she's been working for Diablo ever since Tristam, as he planned to use the Black Soulstone to become the single Prime Evil, and that Leah was to be his new vessel, as she was his daughter. Adria then turns the Black Soulstone onto Leah, allowing Diablo to resurrect once again, this time as the embodiment of all Great Evils into one. Pleased with her services, Diablo orders Adria to leave, so she teleports through a portal while Diablo sets to invade the High Heavens.
Despite her betrayal, Nephalem once meets people whom coincidently knows her, either prior to Diablo's invasion or her betrayal. They reveal that Adria used to be Maghda's friend and part of the Coven. Of course, when Nephalem rescues Adria and Adria later betray them, her torture was likely a mere ruse due to the fact that shemanipulate everyone all along.
Reaper of Souls
After Diablo is slain and the Black Soulstone is stolen by Malthael, the Nephalem goes after him, learning that Adria was the only one who knows where he is hiding. The party goes to the Blood Marsh, where Adria had gone to. They traverse the marsh, fought many monsters that happened to be Adria's creations, and reach the Ruins of Corvus, where they finally corner her.
She tells them Malthael went to the Pandemonium Fortress, but also tells she received a vision from Diablo telling of his return, and that it would be the Nephalem him/herself who would release him. She then throws herself into a pit of blood, reemerging as a Blood Demon and attacking the group, but she is ultimately killed as retribution of what she did.