The next scene in the same chapter as these two, with Sombra and Luna trapped in the realm of the things attacking the empire:

Since the shadows are attracted to regular unicorn/alicorn magic for the purity and light it symbolises, Sombra reluctantly insists Luna use what he’s taught her so far and operate using dark magic. He worried she’s not ready to use so much of it yet, but they really dont have a choice. A bite from one of those things is rapidly overtaking him, messing with his magic.

His concerns are justified after she gets too caught up in the heat of battle and the exhiliration of dark magic, and starts acting obsessed. Unwilling to risk a second “Nightmare Moon” type incident, and hving promsied himself he’d get Luna back to Celestia in one piece, he knocks her out. And legs it towards the only way of escape; the tear in the sky back to thier world…

(Fun fact: The first draft of the fanfic chapter following thier return had her getting very mad at him, complete with full-scale canterlot voice! XD She wasnt pleased he got the drop on her like that, viewing it as embarassing. Sadly it ended up being left out, as i decided on a different idea. The mental image still makes me giggle however…you dont make Luna mad, huh? =P)

Fic Excerpt:

Luna discovered her newest talent when she was almost surrounded. She flared her wings in an attempt to push them back, and was rewarded with several heavy thudding sounds. Blinking, she seen her wingtips were now surround by a weightless blue crystal. She tucked the wings in, and flapped them out again. The shards flew through the air.

She launched herself into the sky, spinning round in the air, sending a wide radius of spikes flying. Touching ground again she sliced another in half with the spear.

She looked over at Sombra, and knew time was running out. He was pale, and even from this distance it was obvious the pain was starting to win. He was laboring to breathe, his distant gaze fixated on the rapidly growing bridge. So she urged her power on, feeling it enveloping her normal Alicorn magic, transforming it into dark.

Then the creatures abruptly stopped, their gazes flicking towards the right. She looked over, and seen a flowing black cloud heading towards them. Two distant yellow pinpricks that looked like eyes. Familiar eyes.

She realized with a sinking feeling that this was what they’d been battling. The over-mind, the consciousness behind all those attacker scouts back in the empire.

“Sombra?!” she said urgently.

“I..I know-!” he gasped, a shudder wracking his body, threatening to make him collapse.

Meanwhile, another wave of those things was marching inexorably towards Luna. He heard her yelling, heard the damage her words did. The sound of thier brittle bodies cracking apart.
He turned from his task a moment, and seen something that sent a chill to his core. The grin on her face, that familiar look in her eyes. She was breathign heavily, eyes dilated to small dots.

–No, I have to stop her!!– he panicked. The bridge was so close now, but he had no choice. Cutting the spell off he turned and ran for Luna, mentally begging her to forgive what he was about to do.

She felt a spell hit her, and before she could form another thought, she blacked out, heavily stunned.
Sombra grabbed her as she fell, nudging her so she lay across his back. Aware of the creatures teeth flashing in the half light as they bit and snapped at him, he turned tail and ran, heading for the almost finished bridge. The approaching cloud, sensing the familiar presence of its long-time foe, screamed with such fury his ear drums ached and his vision blurred. But he forced himself onwards, hoof steps kicking up flares of purple light as he ran along the crystal bridge he’d made, eyes focused on the wavering tear.

Song title is from a song by Black Gryp0n and Baasik - “Fighting Back”. By this point in the fanfic; i found it suited Sombras efforts against the shadow creatures:

Waking from a bad dream into a nightmare,
Stepping out and one again there’s panic in the air
The sky is red and smoke darkens the horizon
I guess once again a force or foe is terrorizing.

My heart stops when the clock stops ticking
Cause I know I’ve gotta help while I’m alive and kicking
I remember days were never black but that’s over
Now that every day a demon’s back
And I’ve had it.

(Interlude)
And so you want to start a war
You don’t know what I have in store
I’ve never felt this way before
And you can’t stop me anymore

I got it from bandcamp, where i seem to spend a lot of time (and money XD) on pone music. Seriously, i love that site!
So yeah, go check the song out, they got some other great ones as well: “Tell Me” is one of my other faveourites, as well as “Getting Stronger”, which is a song from the villains POV! =3 It has Michelle Creber (Applebloom) in it!

“Fighting Back” (c) Black Gryph0n & Baasik
MLP (c) Hasbro/Faust