Luna donned her armour and acquired the power she needed. But she stopped before the dark magic could take control of her, leaving her a pony without dragon features.
This was a request from Fragnostic. Fragnostic wanted to see Luna use Nightmare's powers without becoming Nightmare - "a fusion of duty and power"; "[a picture] to show that it's not Luna anymore but not yet Nightmare [Moon]". Note that Fragnostic emphasised that Luna was not to be evil in the image, and was in fact loved by the ponies and her sister as a whole, having found her role and place in Equestria.
There are some scenes in-show where both of Nightmare's eyes should be visible, but one is hidden by the helmet. After trying to create this, I can see why. It's not easy trying to make the helmet fit. Interesingly, adding the armour to the image makes Luna look a lot smaller.