!King_Amazon!'s Diablo 3 Plot Prediction!
 

I think I've come up with something that I think is at least somewhat feasible, based on this quote from a Barbarian prophecy (from one of the Diablo books, I think):

The question I had was, what is this "final gambit"? This is what I came up with.

People have been theorizing for years that Tyrael could be the bad guy in Diablo 3, based on some reactions from townspeople in Act 5 after you defeat Baal, as well as other bits of information. I was one of the people supporting this theory, until recently, when I saw Diablo in the Diablo 3 trailer.

I have no doubt that Diablo, Baal, and Mephisto will all be in Diablo 3. The above quote supports that, as well. However, the question still remains about what the final gambit is.

Diablo, Baal, and Mephisto have to come back somehow, obviously. We destroyed Diablo and Mephisto's soulstones in Diablo 2, and Baal's soulstone was damaged already, so when we killed him, he should have in theory been gone forever. On the surface, there seems to be no way that they could come back. However, the prophecy says that their defeat is "illusory," so we can assume that they are either not gone, or have returned.

My theory on this: Diablo, Baal, and Mephisto conspired with Tyrael. The Three Brothers wanted the Worldstone destroyed so that the legions of Hell could flood into Sanctuary and into Heaven. However, Tyrael couldn't simply just waltz into the Worldstone Chamber and destroy it without raising some sort of alarm. Tyrael, on the surface, seems like a very helpful and concerned angel, guiding the heroes through Act 4 and "saving the world" in Act 5. This way, nobody really questions his decision to destroy the Worldstone, at least in any sort of hindering way.

What's the conspiracy between The Three Brothers and Tyrael, though? I think they set things up where when the Worldstone was destroyed, they were able to return. This is why Tyrael destroys it, in the end. To free The Three Brothers, as well as break the barrier between Sanctuary and Heaven and Hell. This explains why he claims that the Worldstone was "corrupted" by Baal, and why it must be destroyed, despite what the prophecies said.

This explains what the prophecy above was saying. The defeat of The Three Brothers was illusory, to give people a false sense of security. This is the "Final Gambit." Now, The Three Brothers can not only return, but they can bring the legions of Hell right along with them, and people won't be expecting it.

Well that's my theory, anyway. It could be completely wrong. Tyrael might not be involved in the whole plan afterall. However, I think that he would be able to know that the Worldstone had been set up to bring the Brothers back, or that the Worldstone shouldn't be destroyed since it essentially opens Heaven and Hell up to Sanctuary, which would probably be worse than any sort of corruption Baal caused on the Worldstone.

Deckard cain gets tired of idiots asking him to identify stuff and he turns himself into Diablo

I disagree! ...

Tyrael would unlikely willingly aid the Prime Evils (Three Brothers) as it was he who stopped them long ago during the Sin War (and it was probably he who banished them to their soulstone-y prisons.) When Inarius (generic fallen archangel) created Sanctuary, he, some angels and some demons created the Nephalem - who later became the mortals of the Diablo-world. If we can take the books as canon, it seems that Tyrael has been the one angel that really liked mankind and voted to keep them (I assume the High Heavens wanted to get rid of Sanctuary because of it being created by fallen angels and (risen?) demons.) Tyrael seems to be the one really good guy in all of Diablo, he has aided mankind more frequently than other agents of the High Heavens and as such is considered a "rogue" of sorts.

If all of his benign protector of the human race-thing is a front to keep his true evil scheme of letting the Prime Evils roam the world freely, it would probably have been easier for him to simply not form the Horadrim during the Sin War.

Tyrael might however have been corrupted in some way, like Izual or random and is no longer happy go lucky Angelman. If so, yes - it could be likely that he aid the Prime Evils in some way.

As for the prophecy, I think it only refers to the expansion pack. All of it seems to refer to in-game events:

And a child will cradle Terror in his breast as the heart of man falls under the shadow.
Events of the first game, son of king become host of Terror. Epic hero thrusts soulstone into skull, for unknown reasons.
A Wanderer will pass through the ancient lands trailing chaos in his wake.
The Warrior from Diablo 1 becomes the Dark Wanderer and starts Diablo 2.
The Three Brothers will be reunited as the mortal world trembles before their might.
The Wanderer collects Baal's soulstone and goes to meet his brother Meph.
And so it was foretold that the Three, once reunited, would be shattered again—
Epic Hero #2 kills two of them and one runs away, thus they are shattered.
Their defeat would be illusory—that the final gambit had yet to be played..."
They did not really lose as Baal is still alive, he wants to gambit with the Worldstone.

"And now at last the storm surges forth from the southlands, and the hand of Destruction
Baal lays siege upon the the lands of generic northern guys.
Reaches out to undo the workings of the Ancients. The tides of Hell surge—ready to smash
Baal wants to open the gates for the really huge amount of demons that want to smash.
Down upon the shores of the mortal world — to drown the guilty and the innocent alike.
This will not cause happy times.

A very easy explanation would be that destroying the soulstone (their prison) would merely let the soul/spirit/essence be reborn in Hell, thus the heroes merely freed the Prime Evils from their prisons. This is however not very optimum for the Prime Evils as the Burning Hells are ruled by the Lesser Evils Belial and Azmodan. So they might need to crack some skulls before they can rise again.

Both are very good theories, I haven't fully came up with my own yet but I believe that Deckard Cain (unintentionally) has something to do with Diablo's return. In the Cinematic Teaser, it shows Cain "not feeling well" after he "returns to the ruins of Tristram's Cathedral seeking clues to new stirrings of evil."

So I think that either Cain gets possessed (somewhat like in Diablo II where the hero from Diablo I gets possessed) or maybe Cain is just getting old and dieing.

Well, that's my 2 cents.

Tyreal is looked down upon for helping the mortals because he's not supposed to interfer. Interfering never leads to anthing good as we can see with in D2 and Expac, he is definately bad and everyone hints to it throughout the game.

Maybe after the worldstone was destroyed (maybe it wasn't it could return who knows) and the soulstones Hell finally was able to invade heaven and kill Tyrael's kind? Would make sense that the demons would take out the heavens before the mortal world because the heavens are more of a threat. Except of course for that one mortal (the one you play) that pwns the entire army and it's leaders with a sword.

That being said I wanna see the downfall of the heavens.

Heaven + Hell in Diablo = Epic movie scene.

I think this makes the most sense out of all of your theories.

Thats definately the best theory.