Lilith posed as a human once before during the Sin War, and although I don't think Adria actually is Lilith, I'm open to the possibility she is being controlled and/or is possessed by Lilith. When Inarius last banished her to the Void it is written that he made sure she could never escape and I think that holds true. My theory? Adria, being a practitioner of the Arcane arts, made a connection to the void and opened a pathway between herself and Lilith, and in turn (willingly?) became her host. Lilith's physical form however, remains trapped in the Void.
INARIUS / THE PROPHET
Despite what many claim, The Book of Cain neither directly confirms nor denies that Inarius is the Prophet that enlisted Eirena and her sisters. Inarius using the moniker of "The Prophet" during the Sin Wars doesn't necessarily mean he is the same "Prophet". Perhaps the biggest argument against the theory is that he was imprisoned in Hell at the time, and had been for around 1500 years;
"To this day, Inarius is said to be trapped in Hell within a chamber of mirrors, his eyelids torn from his face as he is forced to gaze upon his misshapen form for all eternity"
That said, I actually do think Inarius could be The Prophet, we just don't have any conclusive evidence just yet. My theory? Inarius was coincidentally rescued when he was summoned from Hell by Eirena's Vizjerei masters. He initially appeared as the aforementioned monstrosity, but changed his appearance before saving the sisters from imminent death;
"My sisters and I waited until all of his associates had gathered to summon a greater daemon.
We slew them all to the last man, but they uttered curses upon us with their dying breaths.
My sisters and I.. we fell. Our deaths seemed certain.
When I woke, the dark figure of the Prophet was staring down at me... That is how I met him."
This picture from the Book of Cain shows a monster being summoned that bears a strong resemblance to Inarius;
Now compare that with this one;
In the book this picture accompanies a passage that describes the original summoning rituals that caught the attention of the Hells. These took place long before the time of Eirena and her sisters, but the similarities are too striking to overlook; the "hood", the upwards-curved head spikes, the three shoulder spikes, the distinctly angelic lower-body clothing. Could it be that this image is used out of context in the Book of Cain?
I also theorize that Inarius' time in Hell transformed him into a vengeful, nefarious being. It could very well be that he manipulated the sisters into being his malevolent tools in the future, with them being "activated" upon their awakening.
<<<POTENTIAL SPOILERS FOLLOW>>>
(This is based on the datamined information regarding Eirena's sidequest in RoS)
It is revealed that her sister Lysa, is in fact evil. She is found in an area called "The Lair of the Prophet" within the Pandemonium Fortress. In this area Eirena is tasked with freeing her other sisters that are presumably being held captive by Lysa.
So if the sister's are vengeful tools of Inarius, why is it that Eirena and those held captive are good?
I'll let the D3 Classic dialogue explain that;
"I feel as though part of me still sleeps in a faraway realm"
"Maybe you need more time."
"That would be nice. But the feeling I have indicates that it is more unpleasant and complicated than that."
"I have thought more on what I am missing."
"Our progress seems to clarify things for me... It is difficult to explain."
"If we keep going, more will come to you?"
"Yes. The spell that sent me to this time was bound to the Prophet. Now he is missing, and some parts of me are missing as well-lost to the ages."
Of course it's also possible that Inarius' intentions were actually righteous and Lysa was corrupted in another fashion. As for Inarius' current whereabouts, I feel like we will learn more with the upcoming Book of Tyrael and Reaper of Souls. Either way, my gut tells me Inarius will play a major role in Expansion pack 2.