Diablo, Mephisto, Baal, Andariel, Duriel, Belial, and Azmodan altogether are possessing Leah’s body with Diablo as the primary one in control as the Prime evil

Leah is Diablo’s daughter and he along with his siblings possess her

Leah is ideal vessel designed to be able to withstand all the power of the seven evils inside her

Diablo planned for her possession since before her birth

Adria is Leah’s mother

Leah is a Nephalem and a witch and archer

Greetings mortal