Author's Note

I missed my deadline to finish this before Eternal came out, but oh well.

Fortress

Just like in Manehattan, it was incredibly easy to follow The Slayer. His path of carnage through the assorted demons was unmistakable.

In fact, while Ponyville was most definitely under attack, there were far few demons than there had been in Manehattan. This meant that The Slayer was able to blaze through them, making a beeline for the treeline of the Everfree Forest.

The forest was far from untouched. Several fires raged through the woods, and the air was thick with the sounds of fleeing animals and rampaging demons. Twilight hoped that Fluttershy didn't hear too much about what happened...

Luna landed and galloped behind The Slayer, expertly weaving over and around the trees. Twilight did the same, but found herself stumbling more often.

Soon, the obstacles cleared, but were replaced by the ground becoming fractured in various places. Despite this, Twilight was able to clear the distance with the elder princess. "So he's going to the entrance to Tartarus, right?"

Luna absentmindedly swung her sword at a demon that was left dazed by The Slayer's attack, cleanly decapitating it. "I would hope so. Still, given the state of these woods, we might have to take a detour by our old castle."

"But does he know that?"

Luna did not answer.

The Slayer stopped just long enough to jump up and chainsaw a flying demon in mid-air. The creature screamed, its two halves falling to the ground just before The Slayer did.

With his breakneck pace now paused, he seemed to notice that the path they were taking now led directly into a gorge, which was also burning. The mountain that housed the entrance to Tartarus was on the other side.

The Slayer looked like he was contemplating jumping down, so Twilight called out. "Wait, Slayer!"

He paused, and looked at her.

She skidded to a stop in front of him, Luna right behind her. "There's an old castle nearby, with a bridge. We can get across there, and make our way to the gate."

He nodded. Twilight galloped by the edge of the ravine, keeping her eyes peeled for the bridge through the remaining trees and the smoke. Luna and The Slayer followed closely.

Soon, she found the bridge. Or, rather, what was left of it. The demons' rampage left it entirely destroyed, the stakes in the ground at either end being the only remaining sign that there was a bridge at all.

Still, this provided the smallest gap across the gorge for them. Twilight turned to The Slayer and Luna. "Okay, we'll have to... carry you across. Is that-"

The Slayer ignored her, and took a running jump across the chasm. The jump was far longer than she was expecting, and The Slayer managed to grab the other cliffside and haul himself up.

"...or you could do that," Twilight said.

Luna couldn't suppress a small chuckle. "We should have seen that coming."

The two of them flew over. The ruins of the old capital of Equestria stood tall, albeit a bit more damaged than before. Curiously, there were no demons in the immediate area, but that didn't rule out inside the castle.

"Alright, it should be just on the other side of the castle," Twilight said. "From there, it's basically a straight shot to Tartarus."

The Slayer nodded and led them again. However, when they got close to the castle, he suddenly stopped and looked at it.

Twilight wasn't expecting the sudden stop, and almost ran right into his back. "Is something wrong?"

Like always, he didn't answer, and walked towards the castle wall. This was the first time they saw him like this, and it was worrying.

"Does he sense something inside the castle?" Luna asked.

"Why would he have interest in our old home?"

Luna and Twilight looked up just in time to see an armored Celestia land by them. She was slightly covered in blood, but her expression made it clear that none of it was hers.

Luna smiled and shared a brief hug with her. "Glad to see you are alright, sister."

Celestia smiled. "Same to you, Luna." She turned back to The Slayer, who was poking around the wall. "What is he doing?"

"We're not sure," Twilight said. After a moment, she asked, "Wait, where's Discord?"

"Right here."

Twilight jumped and whirled around to find Discord lounging in the air. "Don't do that, we're in the middle of an attack!"

Discord smirked. "Oh, sorry, Twilight, but you know that someone around here has to try and keep spirits up."

The Slayer shot a very noticeable glare at Discord upon his arrival, and held a hand dangerously close to his weapon. Discord returned the murderous gaze with a smile. "Oh, don't worry, Slayer," Discord said, "I'm not one of those demons. I'm with the good guys this time."

The ponies nodded in agreement, and The Slayer slowly brought his hand away from his gun. After another moment, he looked at the wall, nodded in satisfaction, and then punched a certain spot with all his might.

The hole revealed a hidden passage that led underground, and he quickly hopped into it.

The ponies were perplexed. "What exactly does he hope to find in there?" Celestia asked.

"I have no idea, sister," Luna said. "But we should probably follow him."

"Do you two not know what lurks below your own castle?" Discord asked, his voice being sincere for once.

Luna used her horn to light the darkness within the passage as she dropped down. "We do, but we know not what The Slayer could be after."

Celestia and Twilight followed her inside. Discord shrugged and slithered after them.

The stairs led downward for quite some time. Soon, Twilight noticed that there was a curve to the descent, meaning they were spiraling further into the ground.

"So... what's down here?" she asked after a moment.

"In the past, treasure and dungeons," Celestia said. "Before..." She gave an apologetic glance at Luna. "Before I left this castle for Canterlot, we had converted it into the original lab and storehouse for our scholars. The same ones that Observable Aegis is a part of."

Twilight tilted her head. "Is The Slayer looking for something that was left down here?"

"That can't be," Celestia answered. "We emptied out this location when we moved labs." Suddenly, her frown deepened. "Unless..."

"As I understand it, there was one artifact that was too large to be moved at the time," Luna said. "We normally sent squads regularly to confirm its condition. According to the records, nothing has changed in all this time."

Discord tapped his claws together as he floated alongside them. "And what was this artifact? I didn't think to dig around your old castle the last time I broke free."

The interior wall of the staircase opened up into a large cavern. Several rows of empty cases and shelves were carved out of rock on the floor below, lit up by magical lanterns that popped back to life at their motion.

The Slayer was far ahead of them, so the three opted to fly straight down instead.

"It was a structure of unknown origin that was buried in the ground beneath the castle," Luna explained. "It was the first artifact we discovered during our rule, and was the reason we founded our group of secret scholars in the first place. To find a way inside."

The lights continued to turn on, until they revealed what looked like another castle, partially buried in the stone. While it looked older than the castle above it, Twilight could see what looked like large, modern glass windows lining a section higher on its wall. Below that, there was an indentation that looked like an entry, surrounded by countless scars and imperfections.

"You mean you never opened it?" Discord asked.

"Nothing we tried could open it, or even damage it in any significant way," Celestia said. "We were even unable to teleport inside. Whatever it is, it's able to suppress magic that attempts to intrude upon it."

Discord snapped his fingers, and nothing happened. He snapped a couple more times, then gave a low whistle. "It even blocks chaos magic. How unusual."

The three of them assembled at the door. "Is this what The Slayer was looking for?" Twilight asked.

Luna looked back at him just as he decided to follow their approach and fall the rest of the way. "It is possible. Though I am unsure that even he will be able to gain access."

The Slayer dusted himself off, then strode towards them. They parted and allowed him to stand right in front of the supposed entry to the building.

Twilight leaned around him to try and see what he was doing. To her shock, one of the red lights on his suit flickered to blue for a moment, and the door slowly hissed open.

The princesses' jaws dropped. Discord's jaw actually left his face and hit the ground with the sound of shattering glass.

Unperturbed, The Slayer entered.

Luna was the first to recover, and followed him inside. "What is the meaning of this?!"

Celestia shook her head and went after her. "Just who is The Slayer?"

Discord swept up his broken jaw and reattached it, then jabbed Twilight in the ribs with an elbow. "Come on, we should go inside."

He glided in, and Twilight galloped after. "Hey, wait!"

Once she was inside, the door hissed closed behind her.

The inside was pitch black, so Twilight lit her horn. She was able to find Discord and the princesses easily enough, since this appeared to be a narrow passage with only one path. Curiously, despite the ancient-styled exterior, the inside was made of smooth metal. It looked downright futuristic.

The Slayer was at the front of the group, and for once showed no haste in his movement. In fact, his head was down slightly, unusual from the way he normally kept it up and scanning for threats.

Soon, the passage ended at a set of steep stairs leading to a hatch. The Slayer ascended and swung the hatch open, and the others followed.

The room they emerged into looked like the bridge of a ship, and Twilight could see the cavern they just left out of the giant window lining the wall behind the consoles. The ambient light allowed them to see, but there was still no light at all coming from inside.

The Slayer walked deeper inside the building, until he was lost in the darkness. Twilight was about to call after him, but there was a sudden noise, like something powering up.

Soon after, interior lights came on, revealing that the building was made out of that futuristic metal, but styled to look like it was wooden. The Slayer was standing in front of a bright blue pillar of energy, which pulsed and spread that same blue glow across barely-visible circuits in the walls, ceiling, and floors.

Celestia was agape. "This... is what we were living above all this time? How old is this?"

Luna was equally bewildered. "Slayer, how are you connected to this?"

The Slayer did not answer, and instead walked past them to the consoles. He tried to press one of the screens, but it turned red at his touch, and displayed a message in a language that Twilight couldn't read.

She walked over out of curiosity. "Is something wrong with this... place?"

The Slayer gave a small nod, then reached into a compartment on his suit. He pulled out a small device, barely bigger than the tip of his thumb, and plugged it into the console.

The screen turned blue and a progress bar started to fill up. When it was complete, it offered a prompt, and The Slayer pressed one of the options.

The lights flickered, and the screens flicked off before coming back on, now displaying an odd circular emblem.

Another progress bar filled up, and once it was done, the screens returned to their previous state, displaying unreadable messages in blue.

Suddenly, a disembodied voice spoke throughout the chamber. "Restoration complete. Hello again, Slayer."

Twilight and the princesses whirled around, looking for the source of the voice. Discord looked equally confused, but was taking glee in that fact.

"Who was that?!" Twilight asked. "Where are they?"

"My memory banks contain a large gap between my deactivation and now," the voice continued. "Would you like me to analyze your suit recordings in order to brief myself?"

The Slayer nodded at the screen he was looking at.

"Very well. Accessing Praetor suit video logs..."

Another progress bar filled up on the screen. When Twilight looked closely, she could even see a similar bar working within The Slayer's visor.

Once the bar was done, the voice spoke again. "Analyzing complete. I am now fully versed in the current situation."

The Slayer turned to the confused ponies and chaos god.

"Greetings, guests," the voice said. "I am VEGA, an artificial intelligence formerly in charge of the UAC Mars base. Due to prior events, I am now assisting The Slayer on his mission."

Luna looked around the room. "And... what would that mission be?"

"The complete and total eradication of Hell and its demons."

"Wait, what prior events?" Twilight asked. "What's the UAC? Mars? What is this place?!"

"I'm afraid we do not have the time for a full explanation, Twilight Sparkle," VEGA responded. "For now, all that matters is that this is a different dimension than our own. This ship was of Night Sentinel design, and the Doom Slayer was formerly a member of their ranks."

"Night Sentinels?" Luna asked. "And The Slayer was...?"

Her questions went unanswered. "Slayer, there is an upgrade to your Praetor suit in storage within the ship. It is the only weapon left in the armory. Would you like me to install it?"

The Slayer nodded. A section of the floor raised and opened up an arm-size protrusion, which he filled quickly. There was the sound of whirring machinery, and then he retracted his arm, and the floor lowered again.

The Slayer's left arm was now sporting a wrist blade. However, when he flexed his arm and extended it, the length rivaled that of a normal sword. He seemed satisfied, and retracted the blade.

"The ship's power levels are low, but I can draw energy from the ground and air around us," VEGA said. "By the time you are done with your mission here, I should have most of the ship's primary functions active, and then we can use this to save Earth."

The Slayer seemed hesitant for a moment, so VEGA continued. "Initial readings show that this realm is similar to Hell because of how it warps time in relation to other realms. Though you were trapped here for over a millennium, only a few years will have passed in our own dimension."

"Cross-dimensional beings, eh?" Discord said. "Quite unexpected, but I like it."

"VEGA," Celestia said, "do you know if our world will remain safe from the demons after we close the portal here?"

"Once the primary Hell entrance is closed, demonic activity should cease almost entirely. After that, it will be up to you to ensure that another portal is not opened, at all costs."

"Then we should get moving," Luna said. "The sooner we leave from here, the sooner we can stop this attack." She started to open the hatch they came through, but VEGA interrupted her.

"That will not be necessary, Princess Luna. I have just enough power now to send you all much closer to the Hell portal. Once the job is done, I will have enough to open a portal for The Slayer to return."

The consoles shifted to the sides of the platform as a set of pylons arranged themselves into a ring in front of the giant window. A walkway also rose out of the floor, and a moment later a bright blue portal formed inside the ring.

"I cannot maintain the portal for long."

The Slayer waved everyone forward, then jumped through. Discord was the next to enter, giggling like a child all the while. Celestia and Luna nodded at each other, then walked through the blue abyss.

Just before Twilight stepped through, she turned around and asked, "VEGA... is this invasion my fault?"

"Judging by recordings from The Slayer's suit, the demons are probably trying to guilt you into assisting them. You were wise to decline, not only because they would betray you, but because The Slayer would not hesitate to kill you if you had sided with the demons."

Twilight shuddered. "Thanks... I think?"

"Please, enter the portal."

"Right, yeah, sorry." She looked back at the portal. "VEGA... will we meet again?"

"Once The Slayer is done here, hopefully you will never have to encounter either of us again."

Twilight took a deep breath, and jumped through the portal.