When Kil'jaeden is finally slain (and I think we can say truly slain, as his death seems to occur within the Twisting Nether,) he seems ready to take the raid, Velen, Khadgar, and Illidan with him, crashing his command ship into the broken, corrupted world of Argus. However, Illidan uses the Sargerite Keystone to create a large portal leading right above Azeroth, giving Khadgar a close enough range to transport everyone safely to Azeroth (specifically somewhere in Azsuna.) Kil'jaeden speaks his last words to his "brother" Velen, seemingly apologizing or at least offering some explanation for his literally diabolic behavior over the last twenty five thousand years. Velen touches his forehead in either a gesture of forgiveness or perhaps just kick-starting the process by which Kil'jaeden explodes in a massive ball of fel energy. It's open to interpretation (on one hand, the Light forgives. But on the other hand, the Light also burns demons away into ash.)
However, there's a big twist:
Having safely landed on Azeroth and escaped Kil'jaeden's explosive death, Khadgar and Velen look up to see that Illidan did not just allow the raid to escape. He brought the entire planet of Argus with them.
And this is the real deal. I haven't been able to play for a while, but apparently once someone on your server downs Kil'jaeden, everyone now sees Argus floating overhead. It even appears that this is true regardless of what continent you're on (except obviously Outland and Draenor.)
Argus is really now nearby. It appears that it is near physically, within the Great Dark Beyond. Azeroth and Argus seem to be twin planets now.
Now, according to I think Chronicle (maybe the Illidan novel) even if Argus isn't in the Twisting Nether anymore, the planet is so saturated with Fel magic that all the same rules vis a vis demons being killable still apply.
Illidan has just pulled the home base of the Burning Legion right up to Azeroth. While that does make us that much more vulnerable, it also means the Legion is much more vulnerable to us (and we know we're going there in 7.3.)
I really want to know what the longterm consequences of this will be. Is Argus just going to be there from now on? Or does this suggest that this expansion is far more likely to see Argus utterly destroyed (thus no longer leaving a giant reminder of a past expansion in the skybox when we move on?) And if it is destroyed, what does that mean for the Draenei, whose plot has centered around their goal to eventually return and liberate their homeworld?