Hollow Shades, night
"Over here!"
Jason hurried over to the whisper, wary of any cultists nearby. He, Luna, and the five guards she had chosen to accompany them huddled around a small bush. The guard who had signalled them moved part of the plant aside, revealing a small hatch, about pony sized, in the ground.
"Excellent work." Luna said. She gestured to three of the guards. "You three, go first. Jason and I will follow, and you two shall bring up the rear. Move out." One of the guards pried the top off of the hatch with their magic, and slipped down it into the darkness below. Two more followed, and then Jason jumped down. Luna and the remainder of the guards entered, and the hatch was sealed behind them.
They made their way carefully along the dark halls, mindful of any roaming cultists. Thankfully, it seemed the guards at the gate were engaging the majority of them, as the place was mostly empty. Suddenly, they heard hoofsteps ahead of them. Luna signalled the foremost guard, and they moved forward. The cultist came around the corner and was snatched from above by the guard, who then brought them back to the group and threw him at Luna's hooves.
"What are you planning?" She asked. Straight to the point. Thought Jason. I like this one.
The cultist spat at her hooves. "Like I'd tell you anything." He said defiantly. Jason walked up behind him.
"Look, you can either answer to her..." He said, grabbing them by their shoulder. He wrenched them to the ground, lighting up his face and staring into their eyes. He held his burning visage close to their own and watched them whimper in fear.
"Or you can answer to me."
"I think its this way." Said Jason. The cultist they'd interrogated had told them about some kind of ritual taking place in one of the larger chambers deep in the cave. He didn't know what it was about, but he told them that it was definitely taking place that night. The guards had knocked him out and hidden him in a nearby room, hopefully he wouldn't be discovered for a while.
The small group continued down the halls until they found themselves emerging into a large chamber. Four unicorns stood in a circle in the centre of it, surrounded by a dark aura. Jason walked up to it, only to be blasted back against the wall by some unseen force.
"Magnificent, isn't it?" The group whirled around at the voice, to see an old stallion gazing at the dome before them. "I've waited so long for this day. My followers have come so far since I came to lead them" One of the guards that Luna had brought with her flew up to the stallion and held their wingblades to his throat.
"What are you planning, cultist?" She snarled. The old stallion merely chuckled.
"Planning? Why, that would imply we've yet to take action. I dare say that what you see before you is the exact opposite. In fact, our plans come to fruition this very night."
Luna stepped forward. "Thy speak of evil most foul as if discussing the weather. What art thy plans, heretic?!" She thundered, slipping into the Royal Canterlot Voice in her rage.
"Ah, dear princess. I don't think you'll be needing any information from me. Why, you'll see for yourselves soon enough. You'll all see." He smiled then, a sick smile that chilled Jason to his core. Jason lit up his hand and held the fireball close to the heretic's face. "Ah yes, the pyromaniac. You do look so much like your brother."
"Tell us what you're doing here, and what you want with Luke." He snarled. "Or I swear to God, I will burn every hair on your body from you one at a time."
At his threat, the old stallion merely chuckled. "My my, such ferocity."
"Jason, I do not believe that we will be able to get any information out of him. He is simply too far gone." Luna said. "Nocturne, if you would?" She said, nodding at one of her guards.
"My pleasure, your highness." They replied, and slammed their hoof into the back of the stallion's head. He collapsed to the floor, still grinning. Jason shivered.
"Well, that guy was fucking creepy." He stated. Suddenly, they heard rapid footsteps from the other side of the cavern. A bipedal figure ran in, clutching a guitar. He was bleeding slightly from a cut on his right arm, and his dark hair hung down in front of his eyes.
Slowly, he seemed to take in his surroundings. He looked first to the dark dome surrounding the unicorns, then to the unconscious cult leader. Finally he saw the ponies standing on the other side of the room, and the flame-cloaked figure standing behind them.
"...Jay?" He said.
"Luke!" Jason cried. He made to run toward him, but as he did the cultists' chant reached a peak, and in unison they slammed their forehooves onto the ground.
The ground in the chamber began to crack, and from the cracks a thick black mist began to seep upward. Jason made to jump over one of them to reach Luke but recoiled from the mist as soon as he touched it, holding his hand as if it were burned. The mist began to coalesce above the four cultist unicorns, swirling and crackling.
The four unicorns suddenly went rigid, and their eyes turned white. Their mouths opened and from them poured a dark, tar-like substance that dripped to the floor and pooled around them. Slowly, their bodies seemed to vanish into the puddles, which rose from the floor and into the dark vortex above them. Jason whipped around, and found that the same thing had happened to each of the Lunar guards. As their bodies dissolved and became part of the vortex, a dark chuckle reverberated through the chamber.
"No," Luna whispered, terror in her eyes. "It cannot be!" The darkness above them stopped swirling.
"Oh, but it can, dear Luna." Said a voice that seemed to come from everywhere and nowhere. It sounded as though there were many ponies speaking at once, all with varying tones.
"No!" She cried. "You were destroyed! Cast out of my mind by the Elements of Harmony!"
"Cast out, yes. Destroyed, no. I still linger."
"Excuse me," Said Jason. "But who the fuck are you?"
"I'm glad you asked, boy. For I am the Nightmare, AND I AM REBORN!"
Suddenly, the darkness shot down onto Luke. He screamed, a hoarse scream that made Luna cringe in fear.
"LUUUKE!" Cried Jason. Luke's body convulsed as the darkness washed over him. It rushed down his open mouth, muffling his scream, and seeped in through his eyes and ears. Slowly, he became surrounded by a cocoon of darkness. Lightning cracked within it, but Luke's muffled screams could still be heard above the noise.