The laboratory wasn't on the most traveled hallways it was deep in the sub-basement’s sub-basement of Luna’s wing. All sorts of strange equipment lined the walls, large gears, and strange bits of strange creatures in jars filled the shelves. In the center was a large vault made of black metal. The door was open,  looking inside was all they saw was nothing. No light, nothing but a void. Luna stood with an earth pony. He wore white jacket and had on goggles, more accurately he had goggles for eyes. but what was more strange about him was he had a false leg. It was mechanical. Acropolis and Rune had never seen anything like it before.

“Good, you’re here on time.” Luna had seemed to drop the royal ‘we’ entirely at this point. “This is Professor Fatuous.”

“Greetings, call me Doc, and no I will not explain the leg.” He took out a flask and took a swig. “Or that.” The lenses on his goggles spun around and adjusted themselves. “Or that.”

“The doctor is head of my, more obscure studies.” Luna explained.

“Admit it Lu-Lu, I do all the really cool stuff, extra dimensional studies, time travel, alchemy, exotic zoology, hard science. As in hard to do.” he gave a deep laugh. “But seriously, don’t touch anything. Who know what inter-dimensional horror might roll on out and eat our consciousnesses.” he gave a chuckle.

“He is here to over see today’s training with me.” The moon princess smiles.

“Yep, i'm here to make sure you don’t scatter your souls across the astral planes.” Doc said that with such a straight face it made Acropolis and Rune very nervous.

“So, what is the training?” Acropolis asked.

“The vault.” Luna’s voice was dark. “This vault-”

“Will take a image from your mind, make it real, and have it try to kill you.” Doc took a swig from his flask. “The trick is to figure out how to stop it.” Luna gave him a very stern look. “Anyway, best sparring partner ever right? Yourself! At least that’s what the original vault did.”

“Original vault?” Rune chimed in. “This isn’t the vault?”

“Oh, no this is the vault, i've just uhh, modified it slightly.” Doc said smacking the side of the vault. “You two will fight each other.” he took another swig from his flask.

“You skills are rather complementary,” Luna spoke up. “Acropolis is physical, and Rune is magical. They are opposites, if you can defeat each other in the vault any pony else you face will seem easy.”

“But, we can't fight each other. I mean i've got more experience with combat. Thats just unfair.” Acropolis questioned, he was becoming more concerned with Luna and her Doc’s sensibilities.

“And you won't really be fighting eachother, this vault is magic you idjit.” Doc slammed the side of the vault. “You could load a thousand ponies in there doing cartwheels for days and they would never touch, believe me I’ve tested it. Now, as I do love adding my personal little touches.”

“Doctor.” Luna seemed concerned.

“I felt that this exercise needed a little, livening up.” the Doctor continued.

“Doctor.”

“So I went ahead and made one more modification.”

“DOCTOR!”

“Lu-Lu, calm down. It nothing dangerous.” He paused for a moment. “Oh, wait no, its super dangerous.”

“Out with it.” Acropolis demanded.

“The tougher you get in reality, the tougher the doppelganger will get.” the Doctor smiled and wobbled over to the workstation and re-filled his flask from a strange container.

“Doc, really must you add these things to an already dangerous thing?” Luna bowed her head in apology to Acropolis and Rune. “I think some other form of training would be-” she tuned into their armors. Their was no fear, no hesitation, they were prepared to walk into the vault and fight the doppelgangers. “Well, it seems they liked your modifications Doctor.”

“Course they did Lu-Lu, I can smell it on them they want to be pushed the brink of oblivion.” His mechanical leg whirred a bit and released a bit of steam. “They want to explore their new abilities. Especially her.” Doc’s goggles slid and clicked as he looked at Rune. “She’s been waiting to test out some of her more, interesting spells.”

“Well then, shall we?” Luna motioned for them to walk into the vault. They proceeded slowly into the void.

“Hello?” Acropolis’ voice echoed through the old house. The windows were boarded up and all the furniture was removed. He recognized it, he was in his childhood home shortly before it burned down. “What in the?” he walked down stairs. In the living room he spotted Rune near the empty fireplace. “Rune!” he ran over to her. “I thought that-” his armor didn't tune in, she was silent, there was nothing in her mind. She turned to face him, her eyes were black, completely black. She grinned an evil and toothy grin at him. “Well this is more disturbing than I thought it would be.” she charged him with her horn trying to gouge him with its tip. He side-stepped her for a few jabs before she caught him under his chest and tossed him through the window boarded up window.

He landed on his back in the crater he fell in as a boy. “This is just getting weird.” he coughed as he got to his feet, she was standing right next to him. She grabbed his leg and threw him into a nearby tree with her magic.

“Come on Acro!” Not-Rune hollered. “Quit being such a little filly!” He quickly rolled back over and lunged at the doppelganger. Landing he swung at her with his wings. She conjured a magic blade just in time to block him. As they traded blows acropolis made to move around to her sides. Their clashing was sending sparks everywhere, scale on magic made such a horrible noise. Acro went in for a strike only to narrowly dodge a bolt from Not-Rune’s horn. He slid off to the side and started deflecting the blots with his wings as he ran around her. Each blast made his wings vibrate, causing him pain. Not-Rune began laughing as her blasts started coming more frequently forcing him to hold his ground. Pulling his wings forward into shields and digging his hooves into the ground he tried to hold his position. As the bolts shook his wings he wondered how long he could hold out against her barrage.

“Hello?” Rune called out in the darkness. Casting a light spell the small shed’s interior came into view. She realized she was in the shed that she was teased in all those years ago as a kid.  “Well, this sucks.” She went over and tried to push open the door. It didn’t budge. “Perfect.” she heard footsteps outside and could see some pony blocking the light that was now filtering through the cracks of the shed. “Acropolis?” the pony stopped and moved to right in front of her, she leaned towards the wall to look through the crack. Acropolis’ wing pierced through the thin sheet metal wall. Rune jumped back just barely dodging the bladed wing as it tore through the wall. She made for the far side of the shed, dodging Not-Acro’s wings as they came through the wall at her. She had run out for places to run pinned on the back wall she tried to figured out where Not-Acro had gone to. All she could hear was her own breathing and the creaks on the old shed.

“Ruuuuune?” Acropolis’ voice crackled from beyond the shed’s walls. His tone was dark and hate filled as he dragged his wing along the exterior wall. “Why don't you come out to play?” His wing came through the wall right next to her head. Panicking she spun around and cast a bolt at the ground blowing away the wall. She ran out of the shed dodging another slash from Not-Acro. She came out in an abandoned playground. The playground was overgrown with barbs and and had not been used in years. Not-Acro rushed her swinging his bladed wings like swords. Dodging his swings was easy for her, she had him on mobility, all she had to do was not let him touch her. She kept her motions fluid and chose to not use her horn’s magic, any second lost could kill her. Knowing full well that she didn't have the brute force to go toe to toe with him. She danced around him, dragging her foot in the mud. Carving symbols in the playground dirt as she dodged the assault. Without warning Not-Acro switched up his attack, he went low and shoulder checked her right out of the circle she was marking in the dirt.

Not-Rune’s assault broke for a moment so she could charge a shot. Acropolis took this chance to make an attack. Running right for her, wings straight out in front he made to cut her down. She pulled her head back, spell charged up enough, and cast it. In that moment Acropolis thrust both wings off to one the side pushing him from harms way. Not-Rune resumed her bolt assault, but this time she couldn't hit Acro at all. Using his wings to thrust around he kept his mobility high enough to dodge the magic bolts. Even if he couldn't fly he could still use his wings for sudden bursts of movement. With one hard thrust he slid behind her. She tried to spin in place to face him, but with one more thrust he was right on top of her. Had she had another second to aim the bolt she had charged would have killed him. Instead it fired off into the trees as her head fell from her body. The spray of blood was more than he had expected, he had never decapitated anypony before. Her pulsating body stood there for several seconds before falling over. The now body-less head of his foe stared up at him. As the static in the back of his got louder.

Landing a few feet away Rune got back to her feet. Her dirt craving wasn't finished and Not-Acro was bearing down on her. Feigning at being injured she tried to limp away. He stopped his assault and began to menacingly follow her she she limped away.

“Aww, what's wrong little Rune?” Not-Acro called after her slowly approaching. “Did I hurt you?” He laughed. While she had his focus on her she grabbed a stick and was quietly finishing her design with her magic. “What’s the matter? Big, bad, Acropolis too much for you?”

Just a bit more. she thought.

“I can't believe you really thought you could defeat ME!” He shouted.

Done.

“Hey Acro,” She spun around showing she wasn't injured at all. “Binden mein Feind und zerdrücken sie in Vergessenheit.” The carvings in the ground lit up with magic. Massive glowing chains burst from the ground wrapping around Not-Acro dragging him the ground. They squeezed tighter and tighter as the chains sunk back into the ground. His screams of agony filled the playground as the chains snapped bone and squeezed him tighter and tighter onto the ground. With one loud and sicking wet crack the screaming stopped. Rune stood staring at the bloody mess that was her foe, the itch in her mind needed more scratching.

“Well done,” Luna’s voice pierced the veil and they suddenly found themselves standing in the vault. “You two did very well, I am thoroughly impressed!” Rune and Acropolis avoided eye contact as they left the vault. “You both displayed great skill in dispatching your foe!” Luna seemed the only pony excited they did so well. To them they had just murdered each other, violently. They could hear the static over the link.

“Hey kiddo’s,” The doctor spoke up, “Don’t let this get to ya, what happened in there wasnt real. Okay? Out here, you two are fine.”

“Luna, I think we need a break,” Acropolis breathed out heavily, “That was a bit more intense than we had bargained for.”

“And slightly more violent that I thought it would be.” Rune added still a little shaken up.

“Of course, you two did fantastic, tomorrow we will do it again,” Luna paused as chimes echoed into the room. “Ah! That would be sunup! You two go rest for now. I will see you tomorrow!”