Rainbow Dash stood slack-jawed at the exit to the bedroom. The grand main hall of the castle stretched before her, and it was much, much larger than she ever could have imagined. The arched ceiling seemed to stretch upwards for almost a mile, and the pegasus could hardly make out that the hall actually had an end. Unable to resist the chance to stretch her wings, she flew up exuberantly and started doing all manner of aerobatics, whooping and hollering the entire time. Her voice echoed throughout the gigantic room, drawing the attention of the many Tartarians passing through the hall.
Twilight sighed and shook her head, though she was smiling in spite of herself. "Good to know she's enthusiastic, at least..."
After a short while longer of tasting the freedom of the open room, Rainbow Dash landed near Twilight once more. She looked all around to take in the ground level attractions. With the castle being as large as it was, there were innumerable side passages littered with shops, residences, and all manner of loitering ponies and Tartarian demons. Twilight motioned for them to continue down the hallway towards a particularly large adjunct room.
This room served as the dimensional entrance to the castle grounds. Twilight explained that the Overlord's castle was linked to other worlds and other locations within Tartarus by means of a magical dimensional gate. Currently, the gate leading to Equestria was sealed on both ends due to an ancient treaty that ended a war between the two factions. Since then, ponies had begun living in Tartarus alongside the demons, but those that grew up in its confines tended to be much rougher around the edges than the typical Equestrian equivalent.
Inside this room, there were many ponies and demons talking amongst each other as they passed into the castle or out through the gate on their daily rounds. Rainbow Dash decided to make a grand entrance to impress her new subjects. Flying up into the air a short ways, she struck a pose and shouted dramatically, "Don't worry about a thing, ponies! Your awesome new princess is here!"
Twilight brought her hoof to her face in utter disbelief as the citizens of Tartarus merely pointed and laughed. Amid the jeering and catcalls, Rainbow slunk to the ground, sheepish and somewhat enraged. "What's going on?! Don't they know to respect their leaders?!"
"That's just it, Rainbow. In Tartarus, the way you earn respect is by seizing power from others. You're a princess, not the Overlord. I did tell you that we'd have to rise up through the ranks before you could properly strut your stuff, didn't I?"
Rainbow Dash cast her gaze askance, her cyan ears falling flat against her head. "Well...it may have gone in one ear and out the other." She perked her ears up and gazed intently at Twilight. "Well, if we're gonna get started, what's the first step?"
"Well, the first step is to get all of your credentials in line." Twilight donned a pair of thick-rimmed spectacles and pulled out a gigantic scrap of parchment that rolled several feet behind her as it unfurled. "We have to get you registered with the Department of Tartarian Affairs. While we're there, we should probably get your newly issued Social Insecurity card. You'll need that if you ever want to apply for reincarnation insurance. Let's see...we'll also need to get you logged as a patient with the Tartarus hospital, since they can fix basically any ailment so long as you pay them the requisite money. They can even bring you back from the dead. That reminds me, we're going to need to get your license to kill-"
"Whoa, whoa, whoa. I'm sure this is all very interesting to you egghead types, but this is all so BORING," Dash interjected, stretching her eyes with her forehooves to emphasize her frustration. "And what's all this about death and killing?"
Twilight sighed in exasperation. "For the last time, I told you you'll have to seize power from others before you'll be recognized as an Overlord. If we're gonna do that, you have to be legally allowed to kill demons and other ponies."
"Kill them?! Don't you think that's a little extreme?!"
"Don't be dense, Rainbow. I did tell you that the Tartarus hospital resurrects ponies who can afford their costs, right? Anypony who can't afford their services probably doesn't have what it takes to survive daily life in Tartarus anyway."
Rainbow Dash tried to come up with a rebuttal to such a grossly cavalier attitude towards life and death, but Twilight dragged her over to the corner of the room before she could open her mouth. Towards the back wall was a smaller gate, very difficult to notice. In fact, if Twilight hadn't dragged her over there, Rainbow felt she would have completely missed that anything was there at all.
"Um, Twilight? What is this gate for?"
Twilight glances around frantically. "Keep your voice down! Low-ranking demons and ponies aren't supposed to know about this place, and we'd like to keep it that way."
With that, Twilight shoved Rainbow into the gate, following closely behind.
It took Rainbow's eyes a moment to adjust to the new room's lighting, but she was somewhat shocked by what she saw. Instead of another huge, bustling portion of the castle, this room was relatively small and cramped. A number of snobbish-looking demons and ponies were seated at an array of desks arranged in a semicircle around a central podium. On the podium, there stood a rather nervous-looking pony addressing the others, making an impassioned yet shaky speech on why he deserved to be able to buy more expensive weapons at the shop.
"Welcome to the Dark Assembly, Rainbow Dash. The pinnacle of democratic life in the underworld. Even in Tartarus, we embrace the principles of solid government."
"Augh, more like the Dork Assembly. Why would I ever want to bother with a place like this?!"
"Well, there's some useful functions that the Dark Assembly serves for increasing what you can do out in the world. In addition, it wouldn't hurt to schmooze up some of the senators, as they are all very high-ranking Tartarian nobility. They have eschewed the lifestyle of an Overlord in exchange for the thrill of public service!"
Rainbow stared at Twilight's eyes, which were sparkling like their owner's namesake. She was about to make another egghead joke when the vote began, prompting Twilight to shush her. Throughout the hall, the senators' voices reverberated as each one delivered a stern "NAY." The pony's expression turned from a nervous grin to an outraged scowl. When the final vote was tallied, not a single senator stood in favor of the proposal.
The pony spat defiantly at the ground and drew his sword with his teeth. "Fine, then! I choose to pass this proposal by force!"
A regal-looking griffon in the front row sighed and removed her glasses. "Very well."
She held one of her taloned forelimbs up and aimed at the pony, suddenly emitting a very bright light. The pony screamed briefly before disappearing. There was merely a pile of ashes where he stood previously. The griffon picked up a gavel and banged it once. "Session adjourned."
Twilight squealed giddily. "Isn't procedure just the best?!"
Rainbow Dash couldn't pick her jaw up off the floor.
"Come oooon, Twilight! Can't we go beat something up?!"
"We've only been at this for like two hours, Rainbow. If you can't be patient for the proper protocols, how will you ever handle yourself once you actually become Overlord?"
"Blah, blah, blah."
Rainbow Dash took a moment to look over her new ID card. It listed her title as "Princess, Pending Bid to Become Overlord." Oddly specific, but Twilight had insisted. "Speaking of which," Rainbow thought to herself. "Hey, I've been wondering. If the ID card specifies a job title here, what does yours say?"
Twilight flashed her card wordlessly. Next to her name, Rainbow saw the words "Rainbow Dash's Right-hoof Vassal, Certified Egghead First Class." She scratched her head quizzically. "What the hell is a...'vase-sale?'"
"That's 'vassal,' Rainbow, and it means I'm your sworn servant. I'm supposed to serve you unwaveringly until such time that you or I die."
"Whoa. I have a servant! That's so awesome!"
"Yes, and I suppose by transitive extension, you also have the Pinkie Squad as vassals, too."
The pegasus glanced around them, suddenly realizing the Pinkies were missing. "Say, where'd they all go?"
"Who knows? You should go find them. I have to take a quick bathroom break."
Rainbow shrugged and flew off in a random direction. Twilight waited until nopony was watching, then pressed a section of the wall, which gave way to her hoof. A small back room opened up, hidden from view, and she stole inside, sealing the door behind her.
In this small, dimly lit room, there lay a book with magical parchment and a feather pen inside an ink well. Twilight used her magic to lift the pen so she could begin writing.
"Enclosed is my first report.
Rainbow is becoming acclimated to life in Tartarus. I'm slowly but surely getting her used to the place, and she already seems to be eagerly embracing her destiny to become the Overlord. The memory wipe worked perfectly, and she remembers nothing of life before awakening in Tartarus, nor does she suspect anything. She knows not of our motives, nor what her true purpose is.
I still question whether this is a good idea. Why did we pick Rainbow Dash, of all ponies? Surely there are deserving candidates for diplomatic Overlords already dwelling within Tartarus? I don't think this is a good idea, but I will trust your judgment. You've never let me down before.
I just hope you know what you're doing.
Your faithful student,
Twilight Sparkle"
With a puff of magic, the letter was sent on its way. Twilight sighed, then exited through the entrance from whence she came.
It was going to be a long day.