The naga of Outland led by Lady Vashj all swore themselves to serve Illidan Stormrage. It's never explained whether or not Vashj had any ties left to Queen Azshara, though it can be assumed that since Azshara sent Vashj to Illidan to begin with, Lady Vashj hasn't really done anything to earn Azshara's wrath. Or perhaps when Vashj slithered through the portal to the shattered world of Outland, she slithered out of Azshara's notice entirely.
Though Lady Vashj and her meddling with the water flow of Zangarmarsh in Outland had little to do with Azeroth, there were still repercussions for her actions. Not everyone followed her blindly, and not everyone approved of her plans for the shattered world. Skar'this the Heretic can be found in the heroic version of the Slave Pens instance in Coilfang, and he is not happy with Vashj or her plans. Skar'this is the only friendly naga in Outland and seeks revenge against Vashj and all the other naga of Outland for their supposed affront to Neptulon.
WARNING: The following post contains spoilers for the upcoming Cataclysm expansion. If you wish to remain spoiler-free, do not continue.
This is incredibly interesting, given what we know of the naga and their relationship to the watery elemental lord, but first things first: Skar'this was the one who could enable players to enter Serpentshrine Cavern, back when attunements were required. The whole reason he allowed this was because he was essentially backstabbing Lady Vashj and really hoping that players would tromp into the raid instance and kill her.
However, this isn't really the case. The problem is that the elements (which you can read more about in a delightful KYL series by Matthew Rossi) themselves naturally progress toward a disorderly and somewhat chaotic state. The elements war with each other because it is entirely natural for the elements to do so. In the case of shaman, they are acting as a sort of channel through which they gently guide the elements into some semblance of order and harmony. In other words, shaman don't really control the elements -- nobody can control the elements. They merely suggest an alternative path towards order that the elements take, if they are agreeable.
In the case of the Elemental Lords, they did not have a shaman to guide them -- they had the Old Gods, and what the Old Gods viewed as "orderly" was anything but. Since the Old Gods pretty much said flat out, "Hey, you guys should war with each other and wreak havoc, that would be totally great," the Elemental Lords were more than happy to oblige -- because that was their natural state to begin with anyway.
When the Titans showed up to put an end to the Old Gods, they also took care of the Elemental Lords, banishing them to a planar prison called the Elemental Plane. It is this Elemental Plane that we get to visit and interact with in Cataclysm. Ragnaros has already broken free of the Elemental Plane, which is why he's a threat in Molten Core -- and it looks as though Neptulon has broken free as well, given his presence in Vash'jir in Cataclysm.
Here's the tricky part that's a little difficult to understand: Fighting for Neptulon, fighting in the Molten Core doesn't make Neptulon a good guy, either. The elements have no real alignment towards good or evil; they are simply there. We are acting against Ragnaros, which Neptulon likes because hey, water hates fire -- but we are doing so because it's best for us. If you want to look at it from a different perspective, it's almost as if we are the elements in this case, and the Elemental Lords are the shaman that are guiding us towards what they would like accomplished. We're going along with it because it's a good idea, as far as we're concerned, and it sounds reasonable enough.
So what does all this have to do with Skar'this and the naga? More complications. Skar'this is supposedly devoted to Neptulon and would like to see his wishes carried out, including seeing that pesky Ragnaros eliminated. During the Midsummer Fire Festival, Skar'this can be found in the Slave Pens, this time freed from his prison -- but his intent is anything but good. He is trying to summon A'hune the Frost Lord, another servant of Neptulon, in order to unleash him on Azeroth. Why? Not because Skar'this has anything against Azeroth, but because Neptulon would like it very much if Ragnaros were gone.
Later, a conversation overheard on a beach in Zoram Strand clarifies things a little further:Loyal servants of the elemental lords, OUR TIME IS NOW.
Too long we have languished in the shadows, biding our time, serving our masters, seeking the end we know must come. Now, as the flames of Ragnaros' Appeasement burn brightly through the night, we have in our grasp the tools to incite war and chaos on a cataclysmic scale!
The Firelord is imprisoned in our world. He is not at his full power, but his might is formidable. Given an equally formidable opponent in this realm, the resulting clash would begin the great elemental war that will bring about the end we have sought.
In Neptulon's service is a great frost lord by the name of Ahune. Even now he is marshalling his power to wage war against Ragnaros. All he requires is a gateway into our world; a gateway we will provide. We have the allies. We have the location. We have the strength and the will.
Final negotiations with our new faithful allies and guests will be conducted in a safe, out of the way location northwest of our primary location in Ashenvale. Before this "festival" of the ignorant masses comes to a close, Ahune shall face Ragnaros in the shadow of Blackrock. The world will quake with the forces unleashed.
All our toils have worked toward this moment. Our masters will rise against one another in one glorious battle that will tear Azeroth asunder.
We live in the end times, my brothers. Hold to your tasks. Strive on. We will soon be triumphant!
The upshot of all of this? Skar'this thinks he's carrying out the will of Neptulon -- and he is, to a degree. But the Twilight cultists aren't asking him favors because they'd like to see Neptulon's wishes carried out. Their real motives are simply that they want to see Azeroth return to a state of chaos and maybe cease to be. They work for the Old Gods. Skar'this has turned away from the naga and only serves Neptulon -- hence his "heretic" title. But Neptulon's ideals and those of the Old Gods are close enough to the same thing that the Twilight cultists are easily able to convince Skar'this to summon A'hune.Ice Caller Briatha: These stones should be the last of them. Our coordination with Neptulon's forces will be impeccable.
Heretic Emissary: Yess. The Tidehunter will be pleased at this development. The Firelord's hold will weaken.
Ice Caller Briatha: And your own preparations? Will the Frost Lord have a path to the portal?
Heretic Emissary: Skar'this has informed us well. We have worked our way into the slave pens and await your cryomancerss.
Ice Caller Briatha: The ritual in coilfang will bring Ahune through once he is fully prepared, and the resulting clash between Firelord and Frostlord will rend the foundations of this world. Our ultimate goals are in reach at last...