Nightmare Moon came forward. It wouldn't have been right to say that she moved, or dashed or rushed or anything else. She existed herself closer to Luna, in the way a house if willing might take a step back when shot. Every bit as unnatural and surreal as it could be, and yet indescribable in any other way. Nightmare Moon wasn't close to Luna before, and she became close to Luna after, and in the process her being undeniably changed position linearly between the two places. To say she moved still didn't feel right after seeing her.
She came to be in front of Luna. Luna was not unprepared, but she couldn't have been ready. She'd known that. She'd been accounting for that. She'd planned around that. None of it made it any less painful when it happened. None of it prepared her for the moment Nightmare Moon's wings, or presumably what had been her wings when she'd been a pony, stabbed through her chest. Luna knew in that moment that she was already dead. No magic or potion could repair the damage done, not that she even had something left to repair where the other had struck. She was living on borrowed time. Her only regret, upon becoming aware of it, was not having enough time to properly appreciate her own demise.
She'd been building up magic as she'd waited for Nightmare Moon to strike. But not for a blow of her own, not quite, and she hadn't been simply amassing power. She'd been working on something more refined, more complex than that. Carefully weaving energy around the structure of the dream and the form itself of her powers, weakening the barrier between deep dreams, reality, and the realm they were in. Carefully forging a connection. It was something she'd once deemed an impossible feat in any useful capacity. She'd thought much the same about the possibility of what she was witnessing with her eyes and feeling within her own body, where her heart had been.
It would still be close to what she'd predicted. An unstable connection, not meant to exist, able to sustain itself only long enough for reality to acknowledge it shouldn't have been there. Not enough to extract anything meaningful from it if not by pure instinct, and certainly not anything that wouldn't have shattered when brought out of the dream. And yet in the circumstances they were in, that pittance was all Luna needed.
As Luna began to die, the bubble she'd created began to shatter. The border between the dream and reality began to fade, and just for an instant the two existed together.
As the dimension they were in and that of true dreaming touched just for a moment, Luna drew something from one to the other. Nothing complicated or fancy, nothing that took too much thought for her straining, fading mind. Just energy. Directionless, formless, it didn't matter. The kind of ceaseless, infinitely bright energy that could only exist in a dream. It would fade in the real world. But it would exist along with Nightmare Moon in the space in between, and that single moment when it would was going to be enough.
Nightmare Moon, for just an instant, was showered in a torrent of power that shouldn't possibly have existed. Alike Luna's blasts in the dream world, and yet real and tangible and there. A silver blast of energy great enough to annihilate anything in its path, enough to turn a mountain range to dust and smoke and still carry on, real for the tiniest fraction of time that could be. The black sphere encasing the two shattered, and a pillar of light surged towards the sky as the dream finally crashed into reality. Then it slowly faded. Luna's body fell to the ground, limp, blood pouring like a river from the hollowed out wound that occupied most of her chest.