I find it really funny that when I click on the hyperlink in the description, Deviantart warns me that "Out there be monsters." It's quite ironic regarding which fanfic you're linking to . BTW, great artwork!
This is a really beautiful and well made picture of Nyx/Nightmare Moon. I can practicly feel the cool night air, practicly see how she's going to move. The colors, the light and shadows, everything looks really well made and beautiful, as stated before.
Yes, some sort of a war machine, ballista most probably. Just had to add something like that to emphasize the contrast between peaceful view with Canterlot and Ponyville to the left and grim castle of Nightmare Moon in an overall bipolar composition of the picture. And her impulse towards... Well, that's obvious.