At the end of Act III, Leah becomes
the new human host for Diablo (and all of his brothers).
What happened to her soul? Is she gone forever, like her father Aidan before her? Does the process of becoming the human host of a demon completely consumes your soul or is there still a possibility to save her?
At the end of the Leah's Journey video from the Behind the Scenes DVD, Chris Metzen hints that Leah might return in the next expansion.
"Her spirit's still out there somewhere. There's a good girl there, that deserves to be saved and brought back from the abyss. So while we don't have any clue yet what subsequent chapters will be, (a couple of clues), I would bet that we would chase that down, because it feels right from a story level and it feels right from a character level."
I don't think she's dead. Even Covetous Shen hints that she might not be lost forever, when talking about "Leah's Soul" (Act 4, The Light of Hope quest).
When Imperius burned Leah and Diablo came out of the shell, I would assume her body was burned away. As she was innocent and unwilling, her soul should continue on, seeing as this soulstone wasn't implanted in her head. Not drawing from lore, but with Adria escaping, there has to be more to the story.
Most likely she's dead.
In the first game, Diablo's host was Albrecht, Aidan's brother, and he's pretty much dead when you see him in the ending.
