Everyone except Princess Cadence huddled into the hospital of the Crystal Empire. Where was King Sombra keeping his empire and all of the ponies in Equestria except the select few who managed to avoid his wrath? It's not like space travel was possible and he marenapped them to another planet.
"Cloudsdale?" Pinkie Pie asked during a conversation they were having across a table, a few books in front of them as they pondered the location of King Sombra. "That's the only town we haven't explored."
"There's also Appleloosa," Twilight added, a book in front of her as she sat at the table reading it. "But I doubt there's anything there that King Sombra would want."
"Not to mention Manehattan, Las Pegasus, Baltimare, and Fillydelphia," Fluttershy whispered.
"Okay, so maybe not the only town we haven't explored," Pinkie Pie blushed.
Twilight slammed her face against the book she had just been reading. "AAGGGHH! What we need is an atlas, not these...what is this?" Twilight closed her book, the title reading Diamond in the Rough. "What the-" she tossed it behind her.
"An Atlas," Shining Armor began. "I don't think there was one in the library. But I think I can go find a map."
Twilight laid her chin on the table in doubt. "No, that's okay, Shining Armor," she said meekly. She put her face into her hooves. "I'm thinking...I know there's gotta be some place we haven't discarded yet..." she pondered and pondered. "I know there's more to Equestria than the everyday towns. But what? WHAT!?"
"Hmm..." Pinkie Pie began. "You remember when your future self came to Ponyville? And you were all panicky about this disaster that never came?"
"How could I not?" Twilight asked into the crystalline of the table. "What about it?"
"Oh, nothing. It's just on my mind."
Twilight moaned. Then...it hit her. "Wait!" she perked up. "The disaster! And the Cerberus!" She sat upright. "TARTARUS!"
"Tartarus?" Fluttershy asked.
"Tartarus!" Pinkie Pie exclaimed randomly.
"Tartarus," Shining Armor repeated. "Oh, boy. You don't want to go to Tartarus, Twilie."
"I don't think I have a choice, Shining Armor."
"Tartarus is an extremely dangerous place. Getting through it is suicide. There's no way you'll even get past the gate."
"I managed to bring Cerberus back to his home in Tartarus."
"You got lucky. And I'm thankful you survived, but there's no telling what's become of Tartarus since King Sombra took power. As if it weren't dangerous enough!"
"Why? Wha-what's in Tartarus?" Fluttershy asked softly.
"Besides acient, evil creatures capable of destroying Equestria, the guardian Cerberus, and the unlimited source of unparalleled magic and power found within the earth of Tartarus," Shining Armor began dramatically. "There's also treacherous terrain, volcanic activity, fire, magma, and if the heat won't get you, the magic will."
"How will magic get to me?" Twilight inquired. "Magic liberates me."
"It is also capable of crushing you like a bug. There's an enormous amount of magical pressure built up in the atmosphere. In fact, so much that if you so much as set hoof into Tartarus, the magic will act as a force of super-intense gravity, and will gradually and brutally crumble you to dust."
Everyone gulped.
"I'm sorry, Twilie, but...I can't let you do this. I'm all for saving the world - you did it right before I married Cadence - but, until now, I didn't think you would ever have to put yourself into unnecessary danger. As your older brother, it is my responsibility to protect you. And I will do that at all costs."
Twilight did not take her brother's consideration towards her safety kindly.
"We don't have a choice!" she argued. "It's either risk our lives or leave Equestria to be consumed by that monster!"
"Twilie..." Shining Armor walked over to her and put his hoof on her shoulder; she dismissed it. "I'd rather have all of Equestria consumed by King Sombra and have us enslaved for eternity than let anything happen to you or Cadence." He paused. "Do you understand?"
Twilight crossed her hooves in a pout. "I understand," she said. "But I don't agree with you."
"Well, unless you have some way of protecting yourself from gravity strong enough to shatter your bones, there's no way you're going to Tartarus."
"What about that magical protection spell you conjured up over Canterlot before your wedding?"
"That, Twilie, was to defend us from mere flies compared to the relentless damage of Tartarus. It will take a lot more than that to protect ourselves from the atmosphere...I'm sorry...but there's just no way around it."
Twilight put her head down on the table. She thought about her options. "What if I am able to conjure up a shield able to protect us from Tartarus?"
"Well...I suppose...but I don't think even you are capable of that...no offense."
Twilight hopped up to her hooves. "None taken; but I know of one pony who is."
"A protection spell, ya say?" asked Star Swirl the Bearded. "I haven't cast one of those since...euh, ever-"
"Did you just say ever-?"
"But, that doesn't mean I can't, Twilight. I'm sure I got something for ya in all these pages." Star Swirl started to search the shelves. "Mighty noble of you to wanna wander off into Tartarus." He flipped through some books, and finally stopped after three or four unsuccessful trials. "Ah, here we are! A plasma shield spell. It will protect you and your friends within a twenty-foot radius. And it's virtually indestructible."
"Virtually? As in...not completely?"
"'fraid so."
Twilight did not reply; she looked down at her hooves.
"It'll be enough to get you to King Sombra: I promise that."
Twilight rose back up. "I trust you," she said boldly.
"Good, good." Star Swirl the Bearded ripped a page out of his book. "Here's the page which explains how to cast the spell. I highly recommend you cast it before you pass the gate to Tartarus."
Twilight took the page with her horn, thanked the stallion, and started back to the Crystal Empire. She did not stop until she was within its borders. She was ready for Tartarus.
"I'm coming with you," Shining Armor said, stepping forward. "I'm not letting my little sister go alone this time."
Twilight accepted her brother's assistance.
"Can we come to?" asked Pinkie Pie.
"Of course. In fact, let's bring Princess Cadence with us, too. I want everypony to give this journey their best - for if we fail...we will have no Equestria to remember."
A moment of silent contemplation.
"Let's go," Twilight said with a surprising lack of enthusiasm.
It was a long, ungodly trip to the gate of Tartarus. The ponies were attacked numerous times by large, black mares who had escaped the realm in a surprise pursuit of attack. They survived every battle - quite easily - but their morality started to decline as they approached the wretched realm of destruction. Twilight sounded bold and confident when she gave her speech, but now, her faith in herself was deteriorating as they grew ever so closer to the heat of Tartarus. Even before seeing the gate, she could tell they were getting close when the temperature gradually rose. Nearing the gate, she and her friends began to sweat. They were practically melting when the gate was in sight.
"There...there...there it is," she groaned. She wiped her forehead. "Now about that spell..." she withdrew the paper Star Swirl the Bearded gave her and studied the spell. "Alright...let's hope this works..." Her horn illuminated, sparks crackling around her, a flash of light, and there was a large, purple shield hovering over the ponies' heads which touched the earth. It followed them in. The gate was supposed to be closed and guarded by Cerberus, but obviously, King Sombra has had his way with it. With only the plasma shield protecting them, the ponies made their way inside, instantly feeling the immense gravity of pressure and magical buildup in the atmosphere, as well as the intense heat which caused them to prespire even more than they were prior to their entrance into Tartarus.
A swarm of black ponies - like a haze of flying monkeys - rose over the horizon mere steps into the environment. They zoomed down to the earth, slamming against the plasma shield and being sent back into the air upon contact, all within the shield safe from harm. But this was only the first few moments into the forsaken land. It would be another hour or so until they would reach King Sombra's chambers.
With the swarm "attacked" and ridded of, they continued on, slowly stepping further and further into the inhospitable environment. Volcanoes surrounded them on all sides. The terrain beneath their hooves became hard, hot, and jagged. Humongous stalagmites created a path through the territory, guiding the ponies to their destination. Each step became more painful than the last.
How could King Sombra ever survive out here? Twilight wondered to herself. Nopony could ever survive a minute in this place!
Then: disaster. One of the ancient creatures capable of destroying Equestria burst from the ground immediately ahead of Twilight. The creature resembled a quetzalcoatl. It was armored in a heavy, black, metallic armor which looked indestructible. It had huge, red, fiery eyes which were used to stare bloody daggers at the small ponies which dwarfed it in size. The first thing it did was slam itself against the shield, the shield reacting, sending it back in the direction from which it came, but, the quetzalcoatl, being extremely resilient, returned to its position, and started casting powerful attacks such as fireballs and lightning bolts, all from its mouth. None of its attacks struck the ponies, but the shield grew weaker. It growled a heinous growl.
Twilight closed her eyes and gulped, fearful of what would become of them if the quetzalcoatl made its next move.
The beast flew up into the skies, spun around several times, and then charged with all its might to the helpless ponies. As it dove, a massive amount of strength built up in its path. When the beast hit the shield - brutally damaging it - it was slingshot backwards into orbit and disintigrated as it left the atmosphere.
"Come on...we must...stay...on task..." Twilight panted. "I'm sure there's...not much farther...to go..."
Just then, a volcano erupted nearby, bringing in a river of boiling lava straight to the mares (and Shining Armor). The lava spread out when it touched the shield, and altogether avoided the ponies. Fearful of moving, Twilight ordered the rest to remain put until the lava passed. It was several minutes before the path through the remainder of Tartarus was visible. By that point - between heat and exhaustion - Twilight was unconscious.