Over WoW's years, we've had the focus bounce between the Old Gods and the Burning Legion as the main villains. Most of the time it has actually focused on the Old Gods - the Legion needs to invade for us to see much of them while the Old Gods have been here for a very long time.
Cataclysm and Mists of Pandaria made Old Gods fairly central to their stories, even though we did not face one directly (though one could argue that in facing the Sha and the Heart of Y'Shaarj sort of counted as facing him.)
With Warlords of Draenor, thing swung back toward a focus on the Legion, and shockingly, Legion is entirely focused on the Burning Legion.
The first raid, which came out last Tuesday (and opens up for LFR today,) is the Emerald Nightmare, which as I've said before, is actually the creation of the Old Gods, despite Xavius being its master and apparently cooperating with the Burning Legion.
Now, this could simply be the fact that Azeroth's history is filled with Old God influence, but there's actually a lot of their presence within the Broken Isles. Let's break it down:
Obviously we have the Nightmare, which was created by Yogg-Saron after his corruption was allowed to enter it via the failed World Tree Vordrassil in Grizzly Hills, and now its corruption saturates the northern part of Val'sharah and the world tree Shaladrassil.
Side note: The Nightborne call themselves the "Shal'dorei." In the various elvish languages, each elvish group tends to use this kind of nomenclature. The odd thing here is that the Night Elves are referred to as "Kal'dorei," meaning "children of night." The high elves (and possibly also the Highborne, as the former are the descendants of the latter) use "Quel'dorei," which I'd assume means "the high children" or just, you know, "highborne." The Blood Elves (who are of course just High Elves, though you could argue that the feeding upon fel magic might have changed them enough to be considered a different race) are called the "Sin'dorei," which I'd assume means "children of blood."
So what, then, does the prefix "Shal" in "Shal'dorei" mean? Perhaps it means darkness? It can't mean Night, because that's Kal. If it means darkness or shadow, it could be connected to the Pandaren word Sha, which of course refers to the manifestations of Old God corruption, aka, shadow/void corruption. All pretty ironic given that "Sha" means "Light" for the Draenei (hence Shattrath City.)
In Azsuna, we deal with the Naga. Azshara was, of course, aligned with the Legion during the War of the Ancients, but shortly thereafter, she teamed up with the Old Gods, who transformed her people into Naga (funny how you rarely get to see any Naga who talk about or acknowledge their history as Night Elves.) Given that it has been ten thousand years since she worked with the Legion, do we really assume that she is now?
Stormheim is the zone where the connection is hardest to make. Helya certainly has tentacles, but her history is really pretty exclusively tied to the Titans. She was the first Val'kyr and used to perform the role that Eyir does, except that she resented it and rebelled, creating Helheim and cursing vrykul to become Kvaldir to serve her instead of being judged and potentially uplifted by Odyn. The way her story is described in Chronicle, this rebellion does not seem to be the product of some Old God corruption, but merely Odyn actually being a real dick and forcing her into this transformation against her will. Still, one could very well ask how a Val'kyr was transformed into the creature she is now, and that might be the work of team purple.
I mentioned earlier that I have my suspicions about the Nightwell and its source. Most of the horrid behavior of the leadership in Suramar can probably just be chalked up to the demonic occupation (so Elisande's regime is Vichy Suramar. Does that make Thalyssra Charles de Gaulle?) Still, those Il'gynoth whispers about finding "him" by drowning yourself in a circle of stars... it's thin, but I don't think it's nothing.
But then there's also Highmountain. I just did the zone on my Warrior (at some point I really need to go do all the side quest chains there - it seems as if there are far more of them there than other zones.) Obviously, this was the home of Neltharion, who got too many whispers from the Old Gods and went crazy, becoming Deathwing. This was all after their imprisonment, so there's not really any reason to think that their influence within the mountain has diminished.
Consider Dargrul - in the face of the Legion invasion, Dargrul rebels against his former allies when they consider lending the Hammer of Khaz'goroth in aid to us outsiders. He then proceeds to become an incredible dick - destroying multiple villages and sending his people to attack almost everywhere. He becomes totally obsessed with wielding the hammer.
Is he just another muscle-headed leader who is obsessed with strength? He could be, but this is also a guy who has been living within Neltharion's lair. Is it possible that instead, he heard some very convincing whispers himself that told him to take the hammer?
Ultimately, though, you have to wonder what the endgame is in all of this - what influence have the Old Gods had on these events and what to they stand to gain?
The latter I think is not terribly hard to interpret from Il'gynoth's whispers. "Five keys to open our path. Five torches to light our way." This heavily implies that the five pillars of creation we're collecting could inadvertently release the Old Gods from their prisons. It's one of those dangers that has always been a possibility in lore of Warcraft.
We are desperate to close the portal at the Tomb of Sargeras, and using these Titan relics to do so seems like a good idea. In fact, it may even work, but with some unintended consequences.
So the Old Gods want us to get the Pillars of Creation so that we will accidentally release them in our efforts to stop the Burning Legion.
That's all well and good, but how the hell could they put us in this position in the first place?
Put on your tin-foil hat, because the pieces are there, if you're willing to connect them with red yarn.
Why is the Legion invading now? Because of Gul'dan. Our own Gul'dan was killed decades ago, but with this new one, they've been able to use him to open the portal at the Tomb of Sargeras.
Ok, so how did we get this new Gul'dan?
We would not have had any duplicate persons if it had not been for the fact that Garrosh Hellscream and Kairozdormu traveled to an alternate universe. Garrosh has the will, Kairoz has the know-how.
But how would they convince these two to do it?
For Garrosh, we defeated him in Orgrimmar while we was literally covered in Old God blood (which apparently has eyeballs in it because gross.) Now, Y'shaarj has been dead for eons, but that hasn't stopped the Sha from existing or the Heart of Y'shaarj from whispering to us during that fight (though I don't remember there being anything cryptic there.) Perhaps the Heart had enough residual sentience and intelligence to point Garrosh in the direction of Draenor B.
With Kairoz, we take another step back. All the dragonflights were severely depowered - essentially becoming mere mortals - in the wake of Deathwing's demise. The Aspects had to channel their full power together to ensure that Deathwing was obliterated. With the Bronze dragons no longer capable of clearly seeing through time, Kairoz became obsessed with regaining that power, and, well, power in general.
On top of that, Magni Bronzebeard performed the ritual to commune with Azeroth as a response to Deathwing's Shattering. He spent several years as inanimate stone, but when he emerged after the Legion invaded, he claimed that he had been speaking with the Titan Azeroth, and it was Magni who told us to seek out the Pillars of Creation. But how do we know that Magni was right about who was talking to him? Generally, you hear voices from the depths of Azeroth, the thing you're communing with is not exactly looking out for your best interests.
So Deathwing, corrupted by the Old Gods into doing their will, pushes Magni and Kairoz into position. The Heart of Y'Shaarj pushes Garrosh to alternate Draenor, giving the Burning Legion access to a second Gul'dan whom they can use.
The Old Gods gave the Legion an opportunity to invade, thus ensuring that the people of Azeroth would be forced to use the Pillars of Creation to stop them, and unwittingly, these heroes wind up releasing the Old Gods.
Guess we'll have to wait and see!