The soothing light from the tip of Luna's horn cast a faint glow onto the dark stone staircase. She walked deliberately, taking each step with poise and grace, though her mind raced and her heart ached within her body. Her teeth clenched onto a ribbon, and from that ribbon hung a large box covered with black velvet, as long as her body and wider than an outstretched wing.
Water dripped from the ceiling as she stepped on the final landing and into the nigh-empty dungeon. Barred cells lined either side of the small room, bathed in a faint red glow. Near the far wall sat a rusted set of metal stocks, accompanied only by the form of a slouching prepubescent unicorn.
Luna set the box down on a dry spot and took a deep breath of the musty air. She cleared her throat. "I understand you received some troubling news over the weekend, dear nephew. I…" She could not remember the last time she had been left without words. "I suspect your parents disobeyed my command."
Onyx looked over his shoulder. Even in the dim red light that sprang from his horn, Luna could make out the dark circles under his eyes. He did not speak.
Luna's voice took on a sharper tone. "They were to tell you only under on my instruc–"
"I asked them."
Luna paused. She walked forward, her hooves splashing in shallow puddles of dirty water. "You… asked them?" She raised an eyebrow.
Onyx turned back to the stocks. His voice was flat, devoid of any movement. "I saw a picture in my history book. His horn was the same. So were his eyes."
Luna stopped behind Onyx and rested a hoof on his shoulder. "My dear, you must understand: you are not him."
Onyx's head whipped back around. "They said he killed ponies! Thousands of them! They told me he always wanted more power." He stood and gestured widely with a hoof. "Can you believe that? I was this big, greedy sorcerer, and I killed ponies, and I wanted more?" His voice dropped, and his head hung low. "No wonder Cadence hates me."
"She doesn't hate you." Luna lifted Onyx's chin to look him in the eye. Her heart felt like it was being split in twain. "None of us do."
"But what if I turn into him? The books said he–"
"The books said Nightmare Moon was an irredeemable monster. Do you think that is true of your auntie?"
"No!" Onyx pulled away. "You're wonderful! And pretty! And you're never mean!" He looked away. "I'm sorry."
"Apologize not, my dear." She nodded. "I understand what it is to fight your nature, to struggle daily against a desire that threatens to consume you." She summoned the old feelings to her heart, the ones she'd tried for years to forget. "All I could think of was revenge. Tell me, dear nephew: what does the other you seek?"
"Conquest." Onyx sniffled and looked up again, the red light of his horn yielding to the blue glow of Luna's. "That's how I knew it was really him. I… I feel it every night. I feel like I wanna run off and beat up somepony and take everything they have. It's like…" He trailed off.
"Like a whisper that never ceases, yes?"
"Yeah." His lip quivered, and he shook his head, and his voice cracked. "Auntie Luna, I don't wanna hurt anypony. I don't wanna turn into him!" He stomped, and tears fell onto the stone floor. "I don't wanna!"
"Then you must learn to control him, and you must never yield to his desire. Only you can truly put an end to Sombra."
He recoiled at that name. "But… what if I can't stop?"
"I will always believe in you, dear nephew, and you need not fight alone. I have a gift for you." She turned and retrieved the box, once again holding the ribbon in her teeth. She returned and set it down between them.
Onyx's head cocked to the side. "Why did you carry it like that?"
Luna smiled but said nothing.
"Okay…" Onyx sat down and lifted the cover. The soft noise of the velvet seemed so out of place in the cold, hard dungeon, but Onyx cared only for the contents. Inside, laid out on purple silk, sat two thin, ugly iron rods crossed and welded together in the rough shape of a sword. The blunted weapon easily measured twice the length of his body. "What is it?"
"Take hold of it with your magic."
After a moment of hesitation, Onyx did as he was told. He concentrated. At the moment his magic touched the iron, the blade shot upward and hovered in front of his face. Before his eyes, the thin rods began to grow. Like the roots of a tree, black tendrils of metal sprung from the handle. They twisted in intricate patterns, forming a magnificent hilt while dazzling rubies appeared on the pommel and guard. Onyx stared wide-eyed as the longer end of the iron rod transformed into a thin, wide blade, colored like obsidian but sparkling like the night sky with a thousand points of twinkling light. The transformation was complete, and Onyx could not tear his eyes away from the sword's majestic beauty.
"Cool!" Taking the sword again into his magical grip, he swung it around playfully. It felt like it weighed nothing at all. "Whoosh! Whoosh! This is awesome!" He turned his head back to his mentor, grinning ear to ear. "Thanks, Auntie Luna!"
She chuckled at his enthusiasm. "You are very welcome. The Starlight Blade will be a loyal companion. From the moment you touched it with your magic, it became bound to you."
"Kay," Onyx said flatly. He continued twirling the sword around, marveling at how, even with its immense size, it weighed no more than the air he breathed. "Noble's gonna be so jealous! Whoosh!" He giggled. "Oh," he said, turning back to Luna. "Do I have to share?"
She shook her head. "You will not be able to. For as long as the sword is bound to you, nopony but you may touch it with their magic."
"Awesome! I'm… wait." He paused mid-flourish. The gears turned in his mind, and his eyes grew wider. He drew a sharp breath, then carefully set the Starlight Blade back down onto the silk. He stepped backwards, eyes locked onto the sword. "Nopony?"
With a slight smile, Luna lowered her head and tightly shut her eyes. The glow of her horn grew brighter, and the air began to buzz with energy. From nowhere, a wind picked up and swirled around the room, centered at the tip of her horn. A dark blue cluster of magic swelled there, calling a similar cloud of energy around the velvet box. The blue cloud grew deeper in color and thicker in appearance as it closed in on the blade. Onyx cowered as an ear-splittingly loud maelstrom of power flowed around the room. With the howl of a typhoon, all the magical energy in the room converged toward a single point, growing denser still as it closed in on the hilt…
...where it immediately dissipated. Luna gasped for breath, and one of her legs buckled. The blade had not moved.
Tentatively, Onyx stepped forward to examine the blade. He blinked at it several times, but not a trace of Luna's magic remained. He held his breath and slowly, carefully concentrated his magic on the hilt. The sword once again rose as if it weighed nothing.
Onyx brought the sword close to his face and examined the blade. His chest tightened, and his head ached. "But, if you're magic doesn't work, I could… What if he makes me…" He saw that Luna had regained her composure. "What if I can't stop?"
Luna nodded somberly. "Even if Sombra forces his way back, even if all that is left of you is a sliver of light, I will always believe in you, Onyx."
Onyx looked back to the Starlight Blade. Dark thoughts flooded his mind, but he closed his eyes and imagined himself cutting them in half with his sword. It seemed strange to him to be thinking that way. He called back happier memories of traveling with his parents or playing in the gardens. Those times that had felt so normal a moment ago now seemed so very, very far away.
Setting his jaw and opening his eyes, Onyx twirled the sword in the air so blade pointed downwards. He bowed his head, took a deep breath, and said in a low, even voice, "I will not fail you, Luna."