At the end of Act 3:
Adria reveals that she's been working for Diablo and that Leah is actually Diablo's daughter. She then proceeds to use Leah as the host for Diablo's "Prime Evil" form. Adria comments that Deckard Cain was the only one who ever got close to the truth.
During the game, there are many books and journals that give subsequent information to the plot. Some of these are from Deckard Cain. Are we given any hints about what's going to happen either through his writings or elsewhere in the game?
Best Answer
There are various pieces of media outside the game that foreshadow the events that come to pass.
The Book of Cain foreshadows some of the events that come to pass in the game,
For one:
Take note of Fate.
The Diablo novel The Order, hints at certain person and a diabolical curse.
In the Black Soulstone cinematic, take note of who Azmodan is speaking to... and what he is taking about...
If you have the Collector's Edition Blu-Ray/DVD there is some commentary about the possible fate of certain characters.
There's still may question that are left unanswered.