‘Consider that your next test.’
With Sombra's words in mind, Twilight slowly approached the door. 'Is this going to be like last time, when you let me fail and then showed me how to succeed?' Twilight asked telepathically.
'No,' Sombra replied. 'You have already learned everything you need to know in order to deal with this situation, one way or another. However you choose to handle this, the result will have lasting consequences.'
Swallowing nervously, Twilight paused before the door and called out, "Who are you? What do you mean, you've 'been waiting for me'?"
"I have been waiting for a being strong enough to free me," the whispery voice replied. "Surely someone as powerful as you can open this door, right? I've been trapped her for so long..."
'I see,' Twilight thought. 'Sombra must want to test whether or not I can get this door open. Well, I don't think it should be too much trouble to...' Her thought trailed off as she remembered something Sombra had told her.
'When you have mastered both Unicorn and Dark Magic, you begin being able to sense powerful sources of magic before they reach you.'
Hesitating, Twilight asked, "How did you know that I can use Dark Magic? Only those that can use Dark Magic themselves should be able to sense that."
"Of course. We promised to serve King Sombra when he freed us, but he failed. We would be happy to make you our queen, instead, if you would open this door," the voice answered in a pleading tone. "Please, we have been imprisoned for so long. For millennia...
However, Twilight did not miss their slip, going from 'I' to 'we'. 'Sombra said I already knew everything I needed to deal with this, but I feel like I'm still missing something...' And then it hit her, something she had read in Sombra's journal.
Mother told me of why I had been sent to the Crystal Empire. I was the only one that could destroy the Crystal Heart, and free the army of Umbrum waiting beneath the city.
Twilight recoiled from the door as the truth hit her. "You're one of the Umbrum, aren't you?"
"What of it?" The voice snapped back, any trace humility or flattery gone. "At least I inherited my powers, rather than stealing what our King left behind. But worry not, my little thief, all will be forgiven if you just set us free."
"And what would you do if you were freed?" Twilight asked, her narrowed eyes revealing that she already suspected the answer.
"We would take back what was ours, of course," the Umbrum replied. "Our freedom, our lands, and our food. Of course, you would be well rewarded for aiding us. I can personally guarantee that you will get what you deserve."
'Well, somepony obviously doesn't know what subtlety is,' Twilight thought, at hearing the voice's sinister tone. Aloud, she asked, "And what about the Crystal Ponies?"
"I already mentioned reclaiming our food, didn't I?" The voice said with a chuckle. "Don't tell me you're actually worried about them? You use Dark Magic. Surely you've noticed how they treat you? Like an outcast, a monster. They treated our King the same way. Surely you want some payback?"
Twilight's reply this time was not in the form of words, but rather magic. Her eyes glowed with Dark Magic, and when the spell cleared there was an amethyst crystal bar reinforcing the magical doorway.
"What have you done!?" Screeched the Umbrum within, its voice sounding muffled.
"What I should have done from the beginning," Twilight growled back. "I learned Dark Magic so I could protect others. I'm not about to let you escape if you just plan to hurt others!"
"Why you little-! You'll pay for this!" The Umbrum howled. "When we escape we will hunt you down and-!"
"ENOUGH!" Sombra roared, shocking the being beyond the door into silence. Turning to Twilight, he said, "You just keep continuing to impress, my dear student. Not only did you successfully manage to deduce the true nature and motivation of the being you spoke to, but you also used the Wrathful inclinations of your Dark Magic to fuel your convictions, rather than let them be undermined and twisted." Grinning, he added in a truly pleased tone, "I would say that you truly passed my little test with flying colors."
Twilight smiled at the praise but her expression faltered when a disturbing thought crossed her mind. "What would you have done if I had failed? If I had accidentally released them?"
"I knew you would not fail," the stallion replied without hesitation. "But for the sake of argument, if you had, then I would have been sorely disappointed in you. However, my people would have been freed as a result, so I stood to gain something either way."
For a moment, Twilight was shocked that he would so casually free a being that openly admitted to thinking of ponies as food. Then she remembered who she was talking to, but that only made her curious. "I don't understand. I thought you were trying to free the Umbrum, so why didn't you want me to free them, Sombra?"
Before Sombra could answer, the Umbrum called out, "King Sombra! Is that you? We thought you had died!"
"I did," Sombra replied evenly. "I got better. Now, why don't you scurry along to Rabia and tell her this, 'She may be my mother, but the Umbrum will be freed when I decide the time is right.' Even if you did escape now, you would only be driven back into your prison by the power of the Crystal Heart, without the cooperation of my student and myself."
"...Yes, your majesty," the Umbrum replied after a moment, the frustration obvious in its voice.
"He's gone," Twilight said after a moment.
"Indeed," Sombra agreed. Raising an eyebrow in curiosity, he asked, "How did you know?"
"I felt... something," Twilight replied, placing a hoof to her chin in thought. "Or rather, I felt something since we got here, and didn't notice until it left."
"Excellent. It seems you are beginning to be able to sense Dark Magic," Sombra said with a grin. "With practice you will come to be able to sense more details about others magic, in addition to other types of magic, and- Is there something wrong, my student?" Sombra's grin faltered when he noticed Twilight staring at him with a determined expression.
"That's good and all, but you never answered my question," Twilight replied, before asking again, "Why didn't you want me to free the Umbrum? You enslaved an entire kingdom for that sole purpose! For that matter, why didn't you do it yourself? The gate to their prison is in your study, which means you found it during your reign 1,000 years ago. So... why?"
Sombra stared at her in silence for a moment before looking away. As he did, Twilight saw an expression cross his face that she had never seen from him before. It was so different from his usual range of amusement, arrogance, and irritation that it took her a moment to recognize it for what it was; sorrow.
"You're right, I could have freed them a millennium ago," the Umbrum King said softly, his normal confidence replaced by a more subdued tone. "If I had, Celestia and Luna would have fallen with ease and Equestria would have experienced a thousand years of despair under Umbrum rule. However, I did not know that at the time. I let my fear get the better of me and could not bring myself to open it. 'What if I opened that door and they were good after all?' I asked myself. 'What if I was the only real monster?'"
Twilight could not help it, she laughed at that. Not in amusement or joy, but rather a short, hollow laugh of disbelief. "After the one I just talked to, I don't think you need to worry about that."
"Indeed, I was quite young and foolish back then," Sombra's usual grin appeared for a moment as he replied, before settling back into a more somber expression. "I only discovered the truth after I was trapped in the ice by the Two Sisters. While I was using my Clairvoyance spell to view the world beyond my prison, I discovered a hermetical scribe who was compiling all the knowledge he could find on the Umbrum. Sadly, most of my knowledge of my own race comes from his research. It was thanks to this that I discovered the truth of my people. Tell me, Twilight, what do you think Equestria would be like under Umbrum rule?"
"I imagine ponies would be slaves like the Crystal Ponies were 1,000 years ago," Twilight replied. Then, she remembered what the Umbrum behind the door had said, and added, "Wait, the Umbrum called ponies food..."
"Exactly. Not slaves; food," Sombra explained. "The Umbrum are a race of semi-corporeal beings that feed off darker emotions. Pain, fear, strife, despair... all feed them and let them grow strong. Ponies, beings practically bursting with emotion, are like walking buffets to them, but only after they have been properly broken down and subjugated by them. In order to free me from their prison, they had to give me a pony's physical body, allowing me to sustain myself on pony food instead. That, combined with my upbringing in the Crystal Empire, resulted in my tyranny actually being a far better alternative to true Umbrum rule. Which brings me to my point; I did not wish for you to release my kin, because even I do not desire the world they would bring. And although they call me their king, the fact remains that I am different, changed. At best, I would be a figurehead, ruling over a ruined wasteland. At worst, an outcast, driven out of my own empire by the very beings that I had sacrificed everything to free. In the end, everything I had done was for naught."
Silence hung over the room as Sombra finished his speech. Twilight had no idea what to say. After a few moments, Sombra cleared his throat and said, "Come, let us return to the room and get some rest." With that, he began to climb the stairs back to the castle proper with Twilight trailing silently behind.
Twilight woke up the next morning to the sound of flesh impacting crystal. Jumping out of bed, she rushed out of the guest room fearing the worst; that something had caused Sombra to attack one of the Crystal ponies. She called out telepathically as she went, 'Sombra? What's going on?'
'It would seem that your brother disapproves of me spending the night in your room.' Sombra replied, from where he was held against the wall by Shining Armor's magic.
Twilight just stared at the pair for a moment before her expression twisted into a scowl of annoyance. "Shining Armor, what the hoof do you think you're doing!?"
"Twily!?" Shining Armor said in surprise, having not been privy to the telepathic communication and thus caught off guard. "I was just-"
"Don't you 'Twily' me!" She snapped back. "Black Knight is my knight! Of course he's going to sleep where he can keep me safe! And besides, it's none of your business who I let in my room, anyway! Now, put him down right now!"
Shining Armor shrank back from his sister's tirade, his eyes widening as hers began to glow with Dark Magic at the end.
Before he could respond, Sombra did, "With all due respect, my lady, I can handle myself." A slight flicker of his magic was all it took to break the hold of the prince's magic, letting him land nimbly on his hooves. Grinning a the other stallion's consternation, he said, "It would seem that you have a problem with me, little prince."
"You know full well why that is," Shining Armor replied with a glare.
"Then, I suggest we settle this like stallions," Sombra's grin widened as he went on. "The Crystal Empire was just selected as the host of the Equestria Games. I propose that we both compete."
"In what?" The other stallion asked warily.
"Why, jousting, of course," Sombra answered. "As the Prince of the Crystal Empire, obviously you would be proficient at a sport well known for having originated here. And while combat, including combat sports, is my special talent, I have never jousted before, but I should have plenty of time to practice."
"Fine, but if I win, then you leave Twilight alone," Shining Armor said.
"That's not your choice to make," Twilight cut in indignantly.
'Don't worry, my dear Twilight. He won't win,' Sombra said telepathically, before replying to her brother. "Very well, but when I am victorious, you will issue a public apology to myself and Twilight."
"Hmph, we'll see who's victorious," Shining Armor grumbled as he stomped off.
As he retreated, Twilight turned to Sombra and said, "I'm sorry, I don't know what's gotten into him. Perhaps, we should skip breakfast and just head back to Ponyville."
"He is your brother," Sombra replied. "I fully expected him to act protective, so it was hardly a surprise. I believe you are correct, though. In fact, the sooner we get back, the sooner I can start preparing. There is a magical event in a few nights that should help me regain more of my power and I may even be able to help you harness it, as well."
Hearing that, Twilight's face lit up with a smile. "The Secretariat Comet! With everything going on, I had totally forgotten that it was passing overhead in a couple of nights! Let's hurry back, I'm sure everypony's already getting ready for it!"
Meanwhile, another was preparing for the comet with Twilight in mind. However, while Twilight was thinking of her friends, this being was certainly not one of them.
"Soon, Twilight Sparkle. Soon the Secretariat Comet will pass overhead and you will pay for humiliating me!"