Why is it you think there are Demons in the Dark Below?
There are forgotten ones, nerubians possibly, and potentially naga - but no demons as far as I am aware.
http://wowpedia.org/Erudax - The Duke of Below
Ywaq maq oou; ywaq maq ssaggh. Yawq ma shg'fhn.
"They do not die; they do not live. They are outside the cycle." This is untranslated in game, but the same text Herald Volazj says. Mindbender Ghur'sha also utters an abridged version of this line on death.
http://wowpedia.org/Ozumat - Fiend of the Dark Below
Affliation: Nazjatar (which some believe is the Naga name for the old god city of Ny'alotha, more on this later)
http://wowpedia.org/Warlord_Zon'ozz - General of N'zoth (an old god)
Vwyq agth sshoq'meg N'Zoth vra zz shfk qwor ga'halahs agthu. Uulg'ma, ag qam.
Once more shall the twisted flesh-banners of N'Zoth chitter and howl above the fly-blown corpse of this world. After millennia, we have returned
Gul'kafh an'qov N'Zoth.
Gaze into the heart of N'Zoth.
http://wowpedia.org/Yor%27sahj_the_Unsleeping
Shkul an'zig qvsakf KSSH'GA, ag'THYZAK agthu!
From its BLEAKEST DEPTHS, we RECLAIM this world!
KYTH ag'xig yyg'far IIQAATH ONGG!
SEE how we pour from the CURSED EARTH!
Iilth qi'uothk shn'ma yeh'glu Shath'Yar! H'IWN IILTH! [2]
You will drown in the blood of the Old Gods! ALL OF YOU!
http://wowpedia.org/Puzzle_Box_of_Yogg-Saron
At the bottom of the ocean even light must die...
The silent, sleeping, staring houses in the backwoods always dream... It would be merciful to tear them down...
In the land of Ny'alotha there is only sleep...
In the sleeping city of Ny'alotha walk only mad things.
Ny'alotha is a city of old, terrible, unnumbered crimes...
Y'knath k'th'rygg k'yi mrr'ungha gr'mula...
The drowned god's heart is black ice...
Have you had the dream again? A black goat with seven eyes that watches from the outside.
In the sunken city, he lays dreaming.
The fish know all the secrets. They know the cold. They know the dark.
The giant rook watches from the dead trees. Nothing breathes beneath his shadow.
Beneath the shadow of the darkened spire, there is no light, no mercy, only void, and the chaos within.
In the Sunken City, he lays dreaming...
The references to the giant rook are a source of controversy - some believe it is Karazhan: a large rook, a forest of dead trees, a large and unused cavern below it. Karazhan in lore is also a nexus for many ley lines in Azeroth, an old god in position there would potentially dominion over all magic: so it's a strategic spot for a master plan, and the previous tenant went insane (and in Azeroth, the chief cause of insanity is old god proximity).
Alternately, there a large, rook-shaped tower in north-western eastern kingdoms over The Whispering Woods. When the high elves first sailed up the eastern kingdoms in lore, they landed at what seemed like a nice forest - but then they all started mysteriously going insane and killing one another - those who fled traveled further north and founded Quel'thalas (blood elf capital). That implies that somewhere south along the coast is a seemingly nice forest which is uninhabited and drives people insane: the one that best fits the description by a mile is The Whispering Woods. In the centre of the Whispering Woods is an old-god sized ring of glowing mushrooms, with a patch of dead (or perhaps recently upturned) soil in the middle of it. It's only accessible by air, not a newbie zone or anything - and there is no content there - apart from a few frenzied bears and diseased deer. There are also 5 wisps which wander the woods, and about once an hour all assemble at the ring of mushrooms: but don't seem to do anything further.
Either are possible IMO.
Ny'alotha is a city of "mad things(people)", who commit old, terrible and unnumbered crimes. In Ny'alotha - "there is only sleep". Then you have the combination of "In the sunken city, he lays dreaming" and "the drowned god's heart is black ice", and "at the bottom of the ocean, even light must die". So, in Ny'alotha - which is possibly a sunken city - there is only sleep, and somewhere under the ocean there is a sleeping, drowned god: probably in Ny'alotha.
http://wowpedia.org/Ny'alotha
Ny'alotha is believed to be a physical place within Azeroth, but is also suggested - given all the sleep stuff - that it is related to the Emerald Nightmare. Remember that since the Naga are fallen Night Elves - any reference to sleeping naga should be read as you would references to 'sleep' amongst the night elves themselves: which pretty much always means the Emerald Dream / Nightmare.
"Y'Shaarj also mentions the forest during the Siege of Orgrimmar. When you'll walk among the black forest, you will see."
http://wowpedia.org/Y%27Shaarj
He is the 'progenitor' of the seven Sha primes.
http://wowpedia.org/File:Vault_of_Y%27Shaarj.jpg
We found his heart under Vale of Eternal Blossoms (Heroic Scenario), Garrosh is going to steal it and put it under his throne room in Orgrimmar.
Before your history began, our empire was vast. We shared this world with our sister kingdoms, Ahn'Qiraj and Azjol-Nerub.
Our Gods were many, and powerful. We mantid worshipped the seven heads of Y'shaarj[sic].
Great was the Old One, and terrible was His wrath. He consumed hope and begat despair; He inhaled courage and breathed fear.
When the usurpers came - the ones you call "Titans" - Y'shaarj was destroyed.
His last terrible breath has haunted this land ever since, but the shadows he left behind are mere whispers of his former glory.
I tell you now, because you have earned this warning.
Your gods are not our gods, outsider.
If the Old Ones ever return, we mantid will once again stand by their side.
The wisest among you will do the same.
So the Mantid claim they are bro's with the kingdoms of AQ and AN - they worship Y'Shaarj - and they will return to Y'Shaarj's aide as soon as he comes back: which is at the very end of Seige of Ogrimmar.
During the battle with Garrosh, Y'shaarj says the following things:
You will rest in Ny'alotha.
When you'll walk among the black forest, you will see.
And remember 'the black goat with seven eyes that watches from the outside', that Yogg mentions? Seems cryptic - until you consider that Y'shaarj has seven heads, each with a single eye: seven eyes. Goats in Freudian-dream-interpretation-nonsense are symbols of vague and externalized threats, they may symbolize fathers, or sexual lechery (probably this one isn't useful). Black-ness is obviously a commonality with old gods / void / evil. So for all the talk of things dreaming, a black goat with seven eyes that watches you - is almost undoubtedly a reference to Y'Shaarj. Also note that that isn't Yogg Saron talking about Ny'alotha again - that's Y'shaarj: during the battle of Orgrimmar - he's referencing a sunken city which is very possibly Nazjatar (or something new, but still sunken).
Also:
Based off the "black goat with seven eyes" quote of the puzzle-box it is possible Y'Shaarj is a reference to H.P. Lovecraft's Shub-Niggurath, who was known as "The Black Goat of the Woods with a Thousand Young". However it is not completely certain if the goat is in fact Y'Shaarj.
So, to me - I see the following. The Mantid will probably respond to Garrosh, who will probably resurrect their god next tier. This could have repercussions for both Ahn'Qiraj and Azjol'Nerub. The Rook in the Woods stuff could be a reference to a Karazhan raid first tier again - possibly now with Old God buried underneath (Y'Shaarj?). Y'Shaarj seems to be more related to the woods, he's a goat - not a fish - and his possible Lovecraft inspiration is related to woods. So we have his heart, we're going to feed him during the final battle - he'll manifest somewhere else in Azeroth - the mantid will become excited.
Now we have to think about what the second and/or third raid tiers. The sunken city, Ny'alotha - and whoever sleeps down there (very probably N'zoth) - if it's Nazjatar - then we have a Naga raid. N'zoth is the old god responsible for the corruption of Deathwing - while this is never overtly stated - Warlord Zon'ozz is the General of N'zoth - that he brought a continent force to aide Deathwing is a pretty good indication who it was that was committing to Deathwing succeeding. Apart from all the references to Deathwing being corrupted, it's pretty obvious it's an old god that did it - when the final battle two battles consist entirely of tentacles.
From what we can generally kind of tell - Old God influence is like an airborne virus - C'thun corrupts anyone in range of him. The titans managed to contain Yogg Saron, but as soon as his glass skylight was shattered he managed (though still down in his room below) to alter the will of Titans up above him. We found the heart of Y'shaarj vaulted up below the Vale of Eternal Blossoms, and Garrosh removes it from the vault and brings it to Orgrimmar: it's exposed to the world now. Which probably means it can go anywhere.
Corrupted Deathwing sank into the ocean - what do you want to bet his corpse ended up near Ny'alotha? So N'zoth has probably been awake now for an expansion, building up and corrupting some Naga / Kraken while we've been off in Panda-land.
Also check out this post regarding the crypt under Karazhan:
" But one time while waiting for raid to begin in Karazhan i stumbled upon the HUGE crypt next t where he and his mother are supposedly buried.Anyway its big and creepy, if you manage to not get lost in the maze eventualy you stumble upon a dark room with hooks and chains adorned with upside down corpses hanging from the ceiling.
So i checked the map and realizing that this part of the crypt was a mere hundred meters distant from the Scythe of Elune mine and that remote church in Duskwood sent chills down my spine.If that place wasnt a home to some ancient evil embodied in dark riders harrasing Duskwood nothing was.
Anyways to get a better look i checked up wow map viewer and to my surprise this catacomb,elune goldmine , 2 dark altars in Blasted Lands and Zul Gurub form a tight triangle, each no more then maybe 200 meter away form eachother.
So ive started thinking, maybe there is an old god inside this triangle? Then maybe those crypts and possibly the mine in Duskwood were built to reach him?This would also explain the corruption of Zul Gurub a bit more."
They always had much bigger plans for Karazhan than they originally implemented - from the crypts they were making but weren't accessible in the final build, to the portal entrance and griffon landing spot at the top of the tower that isn't accessible and isnt connected to anything. Karazhan is clearly a more important and unfinished part of the Azeroth lore - something more than Medivh happened there.
With that said, I really don't want to rule out the Whispering Woods up by Gilneas - it screams Old God.
In all this, I see no connection whatsoever to the Burning Legion - or Demons - and therefore I doubt we'll see a Demon Hunter class. If that's true though, the Next expansion almost has to be Burning Legion / Sargeras + Demon Hunter class: but not yet.
Oh I should mention one more thing - The Forgotten Ones and the Nerubians hate one another. The Forgotten worship N'zoth, while the Mantid worships Y'Shaarj - and it is unclear who the Nerubians prefer. Potentially they are into Yogg Saron, given their uprising when Yogg got loose - or potentially since they are bros with the Mantid - they worship Y'shaarj. Since we know that Ahn'Qiraj worships C'thun - it seems most likely that the Nerubians prefer Yogg Saron - and that while C'thun, Y'shaarj, and Yogg Saron all tolerate one another somewhat: N'zoth is a different breed of old god who wars with all three of them.
I'm not sure where the sticks will fall - but this could potentially pit N'zoth(Forgotten+Naga) and the Twilight Dragonflight (via Deathwing) against C'thun(AQ), Y'shaarj(Mantid), and Saron(Nerubians).
If in all this corruption some of the orcs remain loyal to Y'shaarj - that could also be an issue.
The resulting horde of next expansion will almost assuredly be under Vol'jin and won't openly hate the Alliance anymore: but an orcish faction could split away here.
It's possible, although somewhat unlikely - that all the Old Gods are rising here: from the Dark Below. Since:
"They do not die; they do not live. They are outside the cycle."
Is a reference to the Old Gods, collectively (at least I'm pretty sure). We can't truly kill them, and they don't live (hence all the referencing to not breathing).
The giant rook watches from the dead trees. Nothing breathes beneath his shadow.
Things which are in the circle of life breathe - plants and animals all breathe - but since Old Gods cannot die, and do not live - they presumably do not breathe (despite having maws) - presumably they create their physical forms to inspire fear / terror, not for actual useful function / processing of food / seeing in useful directions, etc. As an organism, old gods are terribly designed, is what I'm saying: but they aren't organic.