Adria has just betrayed you, Tyrael and Leah and channeled the great evils into Leah which included the three Prime Evils (comprised of Diablo, Mephisto, and Baal), and the four Lesser Evils (encompassing Andariel, Duriel, Belial, and Azmodan).

Diablo the father of Leah now dominantly is in control of her and all the powers of the great evils united. Diablo/Leah plans on destroying the light in the High Heavens and Diablo/Leah sends away Adria the mother of Leah and Diablo’s faithful servant

You and Tyrael follow Leah through the portal and you try to talk her down before Diablo twists her form into his own

Your attempts will be futile my possession is complete and Leah is too weak to fight all the great evils coursing through her Leah/Diablo says