"They're not going to break me."
Sombra stared up at the ceiling, tracing the cracks that zig-zagged across the ancient stones. Did they really think that a tiny, solitary jail cell underneath the ground was going to rattle him? After spending a thousand years trapped in the darkness and ice, this new arrangement was looking like the luxury suite at a four-star hotel. Neither death nor cold nor isolation had broken Sombra's will and being the princesses' prisoner sure as Faust wouldn't either. He shifted on the cot and continued to count the cracks, scarlet eyes outlining each and every configuration.
"They're not going to break me."
Sombra was a survivor. He endured the Dravite Hill Massacre, the grueling trials of his apprenticeship to the dark magician Grimoire, a battle with two alicorns wielding the Elements of Harmony as well as his own grisly death. He doubted that this new peaceful society tolerated executions so the princesses would most likely keep him locked up for the rest of his life. Fine, let them try. Even without magic, he was a still force to be reckoned with. He'd wait; bide his time.
"Celestia...Luna...Spike..."
These were the names of those he'd see suffer. Oh sure, Spike was still an infant by pony standards. Sombra wouldn't kill him of course; even he wouldn't go that far. Maybe he would just fatten him up with gems and keep him locked in a box. Besides, the young dragon was kind to him and seemed like a decent person. Sombra sat up sharply.
Now where did that come from?
So what if Spike was nice to him? They weren't friends!
"...Cadence..."
Sombra sighed. No, even though she'd sided with Celestia, he would never harm his own child. He actually had to kind of admire her for standing by her convictions. Plus, she did manage to out think him. No, Cadence was off-limits. Perhaps one day she'd come to regret her decision and join him.
"Twilight Sparkle..."
A lump formed in Sombra's throat. Why was he so infatuated with that mare? She was now the enemy! They went on one date and kissed. So what? He slapped his hooves to his cheeks and rubbed at his face irritatingly. Was it her charming personality? Her intelligence? Strong magical gifts? And why did he tell her to go find another stallion to be with?
Despite the strict model of discipline he tried molding himself into, Sombra had always been a slave to his own emotions. He was weak and weakness was not tolerated, especially not in the grand scheme of things! Whatever this infatuation was, he couldn't let it blind him from his mission. His race demanded retribution and Celestia's death would be their wergild. All Sombra had to do was be patient and let his enemies make the first move.
Laying back down on the cot, Sombra put his forelegs behind his head and closed his eyes. Soon, very soon, he'd have his revenge on Celestia and all of Equestria would be his for the taking. He only hoped that Twilight could see the light by then.
Meanwhile...
The Everfree Forest, a strange crop of trees and deadly creatures, surrounded Ponyville to the south like a shadow, neighboring the quiet town before its conception. Ever since the time of Grogar, it was an ancient place, plagued with bizarre fauna and seething with dark magic, an area that most ponies knew on an instinctual level to avoid at all costs. Only the enigmatic zebra herbalist Zecora dared to brave the forest and even she had the good sense to never stray too far into its ominous depths. The facts about the forest had long since become merged with rumors and that alone left the evil in the untapped wilderness inactive. Until now.
In the ruins of the Castle of the Two Sisters, something had reawakened, something old and very angry. Stirring from her weakened stupor, the swirling, inky-black mist rose shakily from her nest beneath a partially-collapsed pillar. Malice stirred within her mind, a deep-seated hatred towards all living things, especially those meddlesome ponies that forced her out of her host. Craning her head up, the Darkness sniffed with non-existent nostrils, filtering out the myriad of smells until she picked up the right scent. If she had a mouth, she would be smiling, one of sadistic pleasure and dark intent.
Slithering across the dusty and debris-covered floors, the Darkness clawed her way over the stone, the thought of vengeance giving her strength. Several hours passed before she finally approached the castle's entrance and turned her gaze towards the east, detecting much aggression and rage in that direction. It took a moment to inspect her meager form, cursing the state that she'd been left in thanks to those vexatious Elements of Harmony. The Darkness was once a princess, nay, a queen, an epitome of the night itself and all the expansive, jewel-littered blackness that it represented. Equestria would be hers for the taking right now if not for those interloping mares!
But now was not the time for spiteful recollections of her humiliating defeat. The Darkness had a task to perform but first, she needed to regain her former strength and to do that, she needed a new host to fuel her. As a disembodied spirit, she couldn't derive sustenance from physical food and in her case, there was only one kind of nourishment that would suffice: raw, negative emotions. The worst the emotion-fear, anger, hatred-the stronger she would become. That, however, would take time but time she had, if her suspicions were true.
No doubt those fools thought that she was gone forever, destroyed by the purifying powers of the Elements of Harmony. The Darkness could almost laugh at the sheer arrogance and naivety they exuded. With great difficulty, the Darkness began her long trek towards her prey. One day, she would find her perfect vessel and punish those miserable Ponyville brats for interfering in her moment of triumph. Only then would she take out Princess Celestia and Luna and claim her birthright as the sole ruler of Equestria.
Nightmare Moon would rise once again...