For those who dont know, Sanctuary was created by Inarius (an Angel) and Lilith (a Demoness). They fell in love and had babies ;p (ask your mom and dad to know how babies are made, not me. Haha) Nephalem were created, pretty much gods more powerful than Angels and Demons put together. They were un-aware of their offsprings power till later in the Sin War. (which i wont get into details.)
Anywho, everything boils down to this. Inarius, Lilith, Angiris Council and the 7 Evils still existed at this point. (Before we had slain Durial and Andariel).
Everyone is wondering where Lilith is (and with good concern!) but... where my theory enters is...
Where is Inarius? We know he was the one behind the Sin War by creating the Cathedral of Light to oppose the Triune... anyway, trying not to waver here. (its hard). After Jared Cain died.. has anyone ever questioned why Deckard Cain who was not in the horadrim (and could have been?) and now suddenly was interested in it and "beleived" ? the Horadrim were extinct... he "picked it back up again".
I think its far fetched but beleivable, I beleive Deckard Cain is Inarius trying to put a stop to "the End of Days" or whatnot. Trying desperatly to save his offsprings he is trying to fashion a way to intervene without them knowing, in disguise obviously.
What if Tyreal figured it out... ? What if Justice came back for him.
Deckard Cain wasnt burned and torched in Tristram... why ? because he was caged. Caged you say ?! Why would you cage ONE "human" while massacre and enslave the others? To make him watch obviously. Why would it be worse to make say.. Griswald watch ? Because Deckard Cain is Inarius ! It'd be pretty horrid to watch your kids burn and get murdered and tortured.... pretty horrible? Would instill... Terror.. in someone... dont you think? (Yes i'm leading you on to Terror....) Lord of Terror.... :p He was most likely the one behind the burning of Tristram which would make perfect sense, Diablo was trying to instill Terror in Inarius.
Side note, Deckard Cain is... how old now? Perhaps "too" old... but he wasnt done teaching Leah... he had to stay alive just a little longer... teach her a little more.....
This would explain why he would train a human (Nephalem) in the Horadric ways because he knew that it was the key to solving the puzzle and that they would be able to figure it out and stop Evil.
He knew he was going to be detected by the Angels at some point as he was discovered by the Evils. He hurried to teach Leah all he knew so that its knowledge would go on, that the horadrim wasnt wiped out and that the Nephalem still had a chance to fight and defend their rights as a seperate living world. Its own entity.
((I wont get into the details of the reasons why i beleive all the worlds that were created by the Angels and the Evils while they possesed the Worldstone and that when Inarius and Lilith tried, it worked.))
To summerize it, its pretty simple, as Anu tried to divide evil in himself and created Tathamet (which is himself.... Anu and Tathamet are the same entity...) it didnt work. Explosion after eons.... (i'm getting carried away into details again arent i? ugh!)
It worked because it was both "Good" and "Evil" that created the the world. The offsprings had both good and evil within them.
Its Balance. Its why the Nephalem are so powerful and why the world "worked".
It would appear that Inarius and Lilith managed to "End the Eternal Conflict" by making the impossible, possible again.
Combining what "The One" Anu had divided... :)
We are all... "The One"... with a little more good or a little moe bad in each of them.
TL;DR My theory --- Deckard Cain is Inarius.
(I apologize if its hard to read because my ideas are all over the place. I was just too excited :( haha. )
Oh! I almost forgot! I said it first! Rain said it first ;p !!