NB!: "Red text" signifies speech in a different language
Tartarus, the infernal hell hole that serves as a prison for the most vile and dangerous creatures known to the world. Guarded by the tree headed dog, Cerberus, it is said that it's impossible to get inn... or out.
Very little is known about this prison of torment, but some believe that once you pass the gates, you will find yourself in total darkness and at the complete mercy of demons with no way out. Others believe the you will simply fall into a pit of eternally burning, pitch black fire that burn away your very soul. The list goes on for ages, but they are almost all false.
Very few know, but the truth is, Tartarus is split into two realms, or circles would be more appropriate. Firstly, we have the 'Inner Circle', the home and birthplace of most demons. The 'Inner Circle' is spiritual realm were only demons can roam unconditionally and were the souls of sinners come to get their eternal punishment. The only thing separating the 'Inner Circle' from the rest of Tartarus is the wall of pitch black hell fire.
Then there is the second realm, the 'Outer Circle', the physical part of Tartarus. The 'Outer Circle' is a place were survival of the fittest is taken to the extreme, a place were everything is capable of ending your life, be it flora or fauna.
The 'Outer Circle' is inhabited by tree kinds of beings: The demons who have taken a physical form. The Gehennalis, beasts of unknown origin that roam the lands in flocks, packs or by themselves. And lastly, there is the Devil Ponies, the infernal descendants of demons and ponies...
Prologue: Escape
The nights around the Gates of Tartarus is always quiet, but never dead silent. You could always hear the smallest of sounds every now and then, be it a small critter whose curiosity got the better of them, or the snoring of Cerberus...
But today was different, for all sound had ceased. It was as if nothing dared to make as much as a peep... Not even the wind it self was playing its usual symphony.
The silence was not the only thing that was strange... The infamous guardian of the gates was gone. While rare, it is not unheard of that the guardian sometimes wander of away from the gates, and it seems today was one of those unfortunate days.
A sudden sound broke through the silence like a shot from a minotaurian firearm. It was the sound of something hitting metal, and it was coming from the gates... From the inside. Something, was trying to break out...
The sound broke through the world once again, louder this time...
Then a third time...
A forth time...
The time between each burst of sound was getting shorter and shorter.
The sound just kept on repeating. Again and again and again the sound of metal being struck rung trough the world, until finally...
The gates slowly opened with an antagonizing screech.
Once the gates had opened enough to see inside, the sound the screeching gates was drowned out by the sound the screams, growls, roars and other unearthly noised from inside the gates. The sounds would be enough to scare of even the braves of warriors or the most furious of beasts.
When the gates were good and open, a silhouette could be seen exiting the gates at blinding speeds, and just in time as well. As soon as the silhouette was out the gates magical security kicked in and closed the gates with an explosion of sound, silencing the infernal noise from within the gates.
The silhouette looked back at the gates with a hint of amusement in its now visible glowing red eyes with slit pupils. It started moving forward, the sounds of heavy hooves hitting dirt being the only sound around.
"Freedom..." the shadow spoke as softly as its deep voice would allow it. It looked up on the starlit sky in wonder. "So, this is the sky... Such vastness... And those lights, are they the stars?" Its eyes continued to explore the vast darkness above it until it laid its eyes on the moon. Its eyes widened in amazement "By the gods... That must be the moon... It's beautiful" it spoke in the language of demons.
The shadow stood there admiring the night sky for maybe a minute, but a slight thud in the distance brought it back to the task at hoof. Its ears perked, the shadow looked in the direction that sound came from. "Cerberus" the shadow growled. "This is no time for admiration, it's time to go..."
A pair of large webbed wings burst from the shadows back, and in a singe quick flap the shadow was off like a lightning bolt.
It did not need to fly for long before it found something of interest: A potential hunting ground. A large forest that reeked of blood and unnatural magic. The howls and roars of predators and the sound of conflict could be heard.
The sounds, the smells... it was perfect. The silhouette grinned, its razor sharp teeth gleaming in the moon light.
The shadow peered down into the forests. It was decided. The shadow dove into the dark forests, and already it could see its first meal of the evening: A large cat like creature with the tail of a scorpion and wings of a bat.
The manticore never saw it coming.
The outskirts of Ponyville, near the Everfree Forest: Fluttershys Cottage
Fluttershy was trying to fall asleep. It had been a long evening and a lot of the animals had started acting weird after the moon rose. She had spent the better part of the evening trying to calm them down and tell her what was going on, but after a while she had given up and gone of to get some sleep, trusing her animals to go to bed by themselvs.
The animal caretaker had almost reached the state of blissfull sleep when a sound brought her back to the world of the waking and up into a sitting posistion. The sound was faint and far away, but there was no doubt what it was: A manticores roar.
Living close to the Everfree, she was used to hearing the sounds of the various creatures that inhabbited the forest, but the roar that she just heard was different. It was a roar of pain. There are very few creatures in the forest that are capable of hurting a manticore, and the things that can hurt them (like Hydras or dragons) tend to not care about them at all.
"O-o my... A-anything that c-can make a m-manticore roar l-like that is bound t-to c-cause some trouble... I-I hope it g-goes back w-where it came from." Fluttershy said to nopony in particular.
At least now she know what had her animals act so weirdly... And now she wishes that she didn't find out. If it continues I should go get the others she thought, I just hope there is nothing to worry about.
Fluttershy did not easily fall asleep that night.
Author's Note
Well, first chapter up, tell me what you think!
Don't expect the next chapter anytime soon, I'm a master at procrastination and writers blocks... 
Fun fact: I actually used this as a base for a writhing assignment in my English class