Everfree Castle, or as it was once known by the citizens of the once-great city of Everfree, the Castle of the Two Sisters.

It was a grand structure back in Luna's time, but as glorious as the castle itself was, time did nothing to preserve it. Nevertheless, it stood as a monument to the past—reduced to nothing more than old ruins, but defiant nonetheless. The same couldn't be said for the city that once surrounded it, as any and all structures that had been built were either completely gone or mostly buried beneath the earth. All that was left was an overgrown forest saturated with magic that had been twisted and ripened with age, like a rotten fruit.

It had naturalized and more or less infused itself into the forest, causing a strange and menacing effect on the flora and even some of the fauna. It was no wonder ponies stayed away from the Everfree, with its ominous air, ever-present gloom, and dangerous plants and animals. To most ponies, it was simply a place to be avoided at all costs, to Luna, it was a tragic reminder of a millennia she'd never get back, and to Nightmare Moon, it was more fuel to keep the fires of her hatred for the one that wronged her burning strong.

The fact that the forest existed at all was bad enough, but now there was something else here, something that didn't belong—something that wasn't here when Nightmare Moon had first returned from her namesake. She felt its presence immediately upon teleporting back to the castle ruins, and the utter foulness and malevolence of its aura nearly made her retch on the spot. It sucked the air from her lungs, leaving her gasping for breath for a few moments. Eyes wide and alert, the alicorn searched the area for any sign of the source but saw nothing, and yet the cloying aura of that wretched place filled every corner of the castle.

It was far stronger here than it had been when she was standing before the Vessel.

"This doesn't make any sense," Nightmare muttered to herself as she continued to scan the ruined throne room, "this aura is so pungent, it's almost as if a gate to the Black Dimension has opened in this very castle, but that shouldn't be possible, not unless..."

She stopped herself as a thought came to mind.

It would be entirely possible if Skal-Gazaath had managed to take over the Vessel, and there was every possibility that such a scenario could come to pass at any moment.

"Perhaps I should've destroyed her after all," Nightmare mused aloud, "but no, that wouldn't be wise, and something tells me this isn't entirely the Vessel's doing, but then what could be—"

A sudden scream ripped through the forest, piercing the relative silence of the night and sending shivers down Nightmare Moon's spine. It didn't last long and was cut short only a few seconds later, but the effect it had on the dark alicorn was profound. The ripples it sent through the thaumic field made it clear it was no normal scream, and if Nightmare Moon had to guess, it was most likely the Vessel's doing, meaning something had happened.

"Could it... no!" Nightmare hissed, "I was too late!"

There was no doubt in her mind that Nightmare Moon's worst fear since finding out there was another Vessel had come true. She could already feel the thaumic field around her beginning to fray from the constant pressure of the 'Other Side', and that otherworldly scream of agony had only destabilized it all the more. There was still much Nightmare Moon didn't know about the Black Dimension or how it worked, but she wasn't completely ignorant either thanks to Onyx Pie's journal.

She still remembered all of what she'd read and that, combined with her own knowledge of the thaumic field that enveloped all of Equis, told her that if the Black Dimension continued to encroach upon its brighter counterpart, it really would be the end of civilization as everypony knew it. Nightmare Moon didn't know exactly what would happen, and though she'd claimed to be too late, what could she do really? She'd only just returned from a one thousand year imprisonment in the moon, and she'd long since abandoned her research into the Black Dimension before that. Her plan had been to discuss matters with the Vessel and come up with a way to prevent the very catastrophe that was becoming more and more imminent.

Now she had no plan, and not just Equestria, but the entire world would suffer a horrible fate, whatever it may have been. So caught up in her feelings of frustration, fear and helplessness was Nightmare Moon that she didn't notice the thick bright blue tendril squirming and twisting just beneath her vision until it was wrapped around her throat. She let out a choked gasp just as another one joined it, then another, and several more. Before the mare even had time to comprehend the sudden and violent change in her situation, the ropy blue limbs had already wrapped themselves around her tall black form and held her fast.

Her magic flared to life only briefly, her horn sparking feebly before snuffing itself out with a painful crack. She didn't know if it was the instability of the field or something else entirely, but it was clear she wasn't going anywhere, at least, nowhere that she wanted to go. She tried to cry out, to ask what this was all about, to demand the culprit show itself, to scream her fury at being caught off guard and her fear of what the near future held, but her voice, along with her breath couldn't escape through the crushing grip of the tendrils wrapped around her throat.

CLOSE... SO CLOSE... THE THRONE... MY THRONE...

The voice wrapped around Nightmare Moon's mind much like the tendrils did her body. They stabbed at her thoughts and blocked out the rest of her world. She hardly even noticed as more tentacles slithered their way out of the ground around her. The words continued to fill every ounce of her as she was slowly dragged into the waiting maw of the black gate that had oozed into existence and taken the place of the floor below.

MY TIME... HAS COME... HER REIGN... IS AT AN END...

I HAVE WATCHED...

I HAVE WAITED...

FOR FAR TOO LONG...

Nightmare Moon's vision began to fade as the lack of oxygen took its toll, but the voice refused to let her go. Its claws dug deeper into her mind and kept her conscious even as the writhing, pulsing tendrils slowly choked the life out of her. As she sank further and further into depths unknown, she found her scattered thoughts focusing on the one mare she'd sworn her undying vengeance against. She saw that radiant face in her mind's eye and in her final moment of weakness, the rage and hatred gave way to deep sorrow, horrible regret, and a longing for days long since lost to time.

It was when that serene image of the sister she both loved and hated in equal measure began to speak that sorrow, regret and longing were washed away, replaced by horror so profound it robbed her of any remaining thoughts.

And now the wait is nearly over... come join me, sister... let us wait just a bit longer...

Yes... just a little longer...

In that bottomless Darkness, Celestia's warm, inviting smile was the last thing Nightmare Moon saw, her words the last thing she heard before all her senses were swallowed up and lost to the void. Elsewhere in the distance, another agonized cry rang out across the forest, and just for a brief moment, the world itself seemed to tremble.

Author's Notes:

Apologize for another interlude so soon (ha) after the last one, but I only made it as long as I felt it needed to be and it was too short to be considered a regular chapter so here we are. I've already started on the next chapter and there should only be a couple of chapters left after this one.