Without spoiling, all I can really say is it looks like a really interesting fight, and the lore nerd in me is itching to see how this inclusion of out-game media (the Sunwell Trilogy manga) into WoW proper will continue once things like the movie hit. Will we see characters from the comic book?
Seriously, here there be spoilers. Thanks to World of Raids for the video link..
I'm not kidding. If you don't like spoilers, do not pass go, as I promise you there's no chance you'll collect $200.
As we can see from watching, the fight seems to combine elements of the Vaelastrasz and C'thun fights. I'm not sure what's going on when you change to the Inner Void, exactly... is Kalecgos tanking himself in there? Anyone care to hazard a guess? It looks like he's fighting a demonic figure but I can't tell for sure. (This is where Matt reveals he has poor eyesight, I guess.) Somewhat like fighting inside the stomach during C'thun, but with a much different flavor.
Man, I was already excited to see new lore in the game, and now this fight has me desperate to see it.
Anyway, here's the video, enjoy your rampant spoilerage.