It is a she, Lady Sinestra. Currently she is using the exact same frame as Kil'Jaedan it will surely change. She is way to humanoid imo. I have a feeling she is the horrifying Secret. The molten yellow all over her and inside her mouth is the same color as a lot of the molten parts in the room under the last room of the bastion of twilight. So the fact that she survived is going to be the horrifying secret. A great twist from the C'thun theory. However, I'm a little disappointed she is the last boss and not an old god. There so much old god lore and Cho'gall is so involved with C'thun the last boss should be old god related. I'd rather have Lady Sinestra as a boss in another raid.
The gold glow makes me think she must have somehow absorbed the demon souls energy or something.
If she is the horrifying secret boss it will be a slight disapointment, as cho'gall was a follower of C'thun, however if Sinestra is in fact the end boss, it brings a weird lore twist. Perhaps Sinestra is somehow allied with C'thun, or a follower of C'thun. After all it says in her wowwiki page that "She survived, however, and forsook her mate and children, taking on the guise of Lady Sinestra, a name she came to use as a dragon as well", but she still talks about having a master when she is encountered in BC
I know the model is there and everything, but I think it could be this guy rather than Sinestra, Sinestra could just be part of the story or something.
http://www.wowwiki.com/Dargonax
Of corse, Sinestra does seem more logical.
Why is everyone calling her by her humanoid name? Do you call Nefarian Lord Victor Nefarius or Onyxia Lady Katrana Prestor?
Her name is Sintharia.
I thought it to be fairly obviously that Sinestra would be the horrifying secret:
Quote from wiki: "Glowing as brightly as the sun, Dargonax burned away, taking Sinestra with him in death."
It was at this very location that Dargonax was created, and destroyed. The immense magic that was released when he erupted must have merged with Sinestra (or what was left of her anyway). As the twilight dragons are able to leech magic energy, this new creature is likely to have these abilities as well.
The whole Dargonax story happened between TBC and WOTLK - as we were running around chasing undead, the new creature was able to recover and leech energy from the twilight hammer and who knows what ever is down there.
As for the whole name thing; both the book and wiki clearly stated that Sinestra adopted her humanoid-form name: " most believed her to be dead. She survived, however, and forsook her mate and children, taking on the guise of Lady Sinestra, a name she came to use as a dragon as well"
As for who she serves: "Lady Sinestra says: The master will be most pleased with this news. The Black Dragonflight will provide you all that you ask." - That is still fairly obviously the Black Dragonflight. Then again, the master of the Black Dragonfligth (Mr. Chin) is corrupted by the old gods.
Also to go against all C'thun fanboys - C'thun would be in no way possible to crawl to that exact location. - However lorewise there should be another old god near this area (remember the blood stone quests, which was supposed to be Old God blood). Combining this with the screenies of the tentacles in Twilight Highlands - then I would say you will find Old God Nr.3 At that spot.