I always wondered why I felt a weird uncanniness toward Nightmare Rarity, but with this particular creation, I think I know what that is now: She reminds me of Queen Beryl from that old anime, Sailor Moon. She's even got the pointy tiara thingy! I guess it just took a proper humanization for it to finally click.
Anyway, all gushing aside, this looks pretty great. I don't think much more need be said.
Wow. Dat Nightmarity. Some aspects of her (the ones that belong to Rarity herself in the first place) are positively dreamy. I'm sure she wouldn't put on such clothes as her normal self, but well, I'm not at all averse to enjoying the aesthetic beauty of her body they help accentuate here.
The exact way her tunic thing descends along her legs, for example, helps draw attention to her hips and their curves, and the high boots further divert us to that area (and the way they still show her toes helps us mentally continue the lines of her body and better remember that there is a body under them (and what a body it is!)). Likewise, the small opening showing her belly button is, well, small, but it has a considerable effect: it makes the clothes feel even more revealing by aligning with her cleavage like that. Said cleavage is also quite impressive in terms of how the clothes are built to focus our eyes there: the collar is done in a way that almost made me miss it at first, as it acts like a great frame for the area of her body we see within it: the head, neck and chest, and that's where it directed my eyes; the way the collar spreads out near its bottom and, together with the breast cups of the tunic, leaves much of the breasts' tops open, and the way said cups are not joined and leave a straight line of exposed skin down the middle further channel our attention exactly to this area. And the collar around her neck (which I still find a fine idea in her pony form) helps accentuate both the areas below and above itself. Now, speaking of the face, you have definitely managed to do something I really like to see in Nightmare Rarity pictures and show both decidedly Rarity features and impressions and add the unmistakable feeling of the Nightmare. I can see Rarity's normal face there all right, but also the way it's made more cold and cruel. Also, the crown of hers is rendered very well, I really like its texture and detail here, and I'm also quite impressed by how those earrings look.
Now, on to the rest. The hair is just stunning, seriously. You've actually managed to make it look properly hair-like by putting in all the little shifts in it that make individual hairs and strands apparent and turn it from a sheet of material into what is evidently made up of countless individual elements. And the way you made it flow is amazing, it both reflects the way her pony form's hair is like and fits this human one.
It's a great idea with the staff here. And the floating cutie mark diamonds. She is indeed a sorceress type, and I like the idea with cutie marks appearing to signify magic in that comic arc, so here it works beautifully. And the dark flame-smoke-like (yes, two in one) thing around her diamonds both shows that magic and the fact that it's my poor darling Rarity's body captured and controlled by the Nightmare.
But the most impressive part, the thing that makes this picture great is the painstakingly precise and detailed shading. Without it, the tunic wouldn't have felt like cloth, and her body would not have been nearly as aesthetically pleasing to contemplate. The gradual and soft, yet not overly soft shifts of tones make it feel lifelike. It's particularly evident along her elbows and wrists, in the area of her lower body where the tunic is bunched up and in that wonderful, wonderful hair. Plus, you actually make said cloth look like cloth and follow her body's lines, not look like a flat superimposed picture. And there don't seem to be any problems with foreshortening, either; the arms and legs and everything look comparatively well-proportioned for the pose and the angle. Finally, the combination of outlines and shading works very well here; you wouldn't have achieved nearly the same result if you used either exclusively. Similarly, the use of sharply shifting blues in the gems of her throne is impressive. It makes them look just great. And that gleam they have is lovely. Now, the only thing I could possibly see improved is the bit on her neck. The sinew/vein/whatever there looks great, but one side is desaturated like her elbow and the other is solid. If you went for either of the two, it'd be much better.
But otherwise, this is a wonderful picture. All the time and effort you put into it really does show here, believe me. Pardon the length of the reply, I just felt like pointing things out precisely.
And when Rarity is rescued, I'll see about offering to give her a massage. She'll have earned one or twenty after this ordeal, after all.
The two sinew/vein/something parts in front, between the collarbone and the cloth collar higher on her neck. See how one is desaturated while another is solid black? Just make both either one way or another, and it'll work much better.
In a way. You did change it for the better. The left neck bit became a bit more defined, the right one became a bit less saturated, so it's good. Also, was the skin-colour tone addition on the collar intended? It would make sense if the cloth is reflective, but it worked well without. Now, if you could add a bit of black linework to the left sinew thing (I should really learn the proper name and definition for it), that'd make it just perfect.