To begin with - thanks for the kind words @rgdraw and @Bruck

(Insert smooth transition) Illustration sketches:

Art War 3 Sketches_4 Illustration thumbnails.jpg2648x2281 2.23 MB

Some thoughts:
My personal favorites are 1 and 4 (which are almost the same thing anyway, so I'll combine them ;D). I like the tighter composition and feeling of grandeur in 1 and the action and wings in 4, so I'll be trying to fit the dangling guy in 1 so that he doesn't obstruct the main character.

2 feels a bit generic and lackluster (too much empty space).
3 is a backview of 4 and I don't think the back would showcase the character too much (even though it's a bit more ominous).
I tried a couple of more crouching poses (as if she's lunging or screeching at enemies), yet it didn't resonate too well with the whole elongated sillhouette I'm going for so those were completely scrapped.

Here's the rough 'victim placement' exploration. I'm also thinking about breaking the symmetry of the character a bit by removing/changing one of the pauldrons and possibly giving her a mech/shadowy arm (nothing too crazy) but that's yet to be tested.

As for the sketches at hand - I'm leaning towards 3, but opinions are welcome

Art War 3 Sketches_5 Illustration 1 Exploration.jpg2160x2903 2.49 MB

Finally managed to work on the illustration some more.
Started thightening the perspective a bit (not too much yet :D). I decided to make the cathedral background a little assymetrical, as it felt too mirrored, but I'll see where it goes when I actually start fleshing it out.
Also removed one of the pauldrons and tweaked the pose for a better flow (and hopefully a better telegraphed condescending expression). The sword feels too bright and messes with the focal point so that goes on the to-do list as well.
Proper material rendering coming soon (Also have to decide on the horn design)

Art War 3 Illustration_WIP1.jpg2160x2948 2.73 MB

Thank you for the kind words!
@nevs38 that was a nice read and is pretty much what I'm going for, thanks

I decided to go back to the concept to update it a bit and check the sillhouette again. I think I could make the spikes on the shoulder a bit bigger but I'll decide in the illustration. Also attempts were made on a logo... it could use some more work

Art War 3 Sketches_6 Blackout concept.jpg2378x2643 2.04 MB

@PeterH thanks ^^ and yeah, that was my feeling as well. At least I'm mostly happy with the overall look and feel, so accentuating elements and some proportions to make her stand out won't really interfere too much with the illustration.
So here's a version with some bigger spikes and added smaller elements all around, tried making it completely smokey below the waist (like a ghost, but I didn't like it, I want to retain the human inside)

Art War 3 Sketches_7 Blackout Bigger Spikes.jpg2378x2643 2.18 MB

Started rendering the illustration, so here's a progress shot

Art War 3 Illustration_WIP2.jpg2160x2948 2.7 MB

Ok, after a break from looking at the character, I decided I can push the shapes even more and add some more flavor elements all around. At long last I'm kind of happy with how the design and the thumbnail sillhouette look.
I'd be happy with some critiques, as I've been looking at her for too long
(Left -old; Right - new)

Art War 3 Sketches_8 Blackout Final design.jpg3266x1922 2.4 MB

And another illustration WIP:

Art War 3 Illustration_WIP3.jpg2160x2948 2.68 MB

Yeah, you're right I'm planning on making it just 'dark' instead of black but I was still playing around with the design and the focal area (head and torso), general hue adjustments are next on the list. Thanks for the heads-up!