"Discord, if you meant this as some sort of funny prank, I'm afraid I must disappoint you, I fail to see the fun part" - Celestia said evenly.
"Moi? I would never do such a thing. I'm reformed! And what would Fluttershy say? Look, I know it is shocking. Can you even imagine what I felt first when I sensed the presence of him next to Fluttershy? But to my surprise, he didn't act violently, at least not to her and her pet rabbit. Yeah, he killed two Manticores, but those beasts wanted to eat Angel and attacked him also."
"Anyway, wouldst thou care to enlighten us unworthy, oh mighty Savant of the Ancient, of this fearless warrior, the Destroyer of Worlds?"
"My-my Twilight, very irony, so sarcasm. Wow! Two years pass and dear ol' Tia here will have nothing over you, I assure you. That's why I am here, to enlighten you, despite your and Tia's best efforts of sabotaging my attempts."
"Then spill it already!" - Luna shouted impatiently.
"Well well well, are you an impatient one. Centuries come, centuries go, little Luna still makes the show! Anyway, if and by any means, you let me tell what I know without unwanted interruptions, I would be more than happy to tell you about him."
"We are listening. But if this IS a prank, you will be placed under the Guard's vigilant eyes for undefined time, even if we won't turn you to stone" - Celestia stated.
"I wouldn't dare to joke with things threatening to destroy your empire of friendship and magic.
It started a long time ago, in a galaxy... scratch that, in a different universe far far away. I don't know much about the creation and the beginning of Hell, but I have had my fair share of horrible experience with it. It all started 47633 years ago. Planet Equis was not inhabited by ponies at the time. I was bored, so I was wandering between different universes without a goal in mind. It was horrible, to be unfit to any of the universes, to not have any purpose, but still knowing about my immortality.
That's when the whispers started. As a being of Chaos, naturally I thought that my personality had started to split up, and at first I found it even amusing. At least, after a long time, I finally had a buddy I could talk to. By the time I realized what had happened, it was already too late. That voice in my mind urged me to commit more and more harmful pranks to the living. Then on one fateful day, it urged me to provoke a war between the sharks and the dolphins on Aquatica, a magical planet consisting almost entirely of water. While I loved wreak chaos onto those silly creatures by changing the color of the water, or even messing with the currents, my original self detested the idea of causing a war. I tried to overpower the other personality, finally, in my desperation, I even tried to get rid of it, only to find out that I was unable to, since there had never been any other personality. As it turned out, my mind became corrupted by demonic forces which I couldn't match, not even with my innate Chaos magic. Eventually the corruption took over my mind, the war was started and the civilization of Aquatica was ruined. Then, serving my purpose for the demons in that realm, they hunted me down and painfully transformed me into what demonkind calls an Imp. Imp is a lowly position in the ranks of the unholy ones. I was forced to serve higher demons as well as to fight at the front lines while conquering other realms and absorbing them into Hell.
For millennia I have survived. Made others sacrifice in the name of Hell's prosperity. I have fought, I have destroyed, I have killed, I have burned, I have been injured by enemies. Then he came. One of the worlds demonkind managed to absorb into Hell has unleashed the bane of their very existence. The invasion of Argent d'Nur. The birth of the Doom Slayer. It is grotesque, in a way. The very man who ripped my body apart with his bare hands gave me back what the demons had taken from me. After my demon body was killed, my essence was set free and I regained my freedom. That happened 3809 years ago. Upon my newfound freedom, I couldn't find the courage to face the man who gave me back my freedom. For once he had been a kind-hearted man, the loss of his loved ones and the invasion of his home planet turned him into the deadliest, most powerful, most merciless, most fearsome entity ever to set foot in the known multiverse. Even the mightiest and most dreaded demons feared his name. For as the demons were the Legions of Doom, he was given the name most feared in Hell by the demons themselves: the Doom Slayer.
Unbeknownst to Equestrian history, I managed to break in to Hell a second time. Though this time, I was in control. I made sure I couldn't be spotted by demonkind. I couldn't interfere, but I could observe. To my surprise, Hell was sorely underpopulated at the time. Vast ranges of the Hellscape had been rid of any inhabitants, only blood and demon corpses left behind. During my wanderings, I have reached a region known by demonkind as Kadingir Sanctum. There, I found three enchanted stones and finally a cursed sarcophagus buried beneath the ruins of a huge temple. The sarcophagus was trembling with dark arcane magic. At that moment, I felt tiny and helpless against the powers lingering in the dark corners of the multiverse. The power of the enchantment was unparallelled by anything I have ever seen or sensed - even until now. Then I saw the mark on the sarcophagus. It was the mark of the Doom Slayer. Unbeknownst to demonkind, I have managed to extract the information contained within the mark by prodding it with Chaos magic. I recognized that the magical samples of the three stones were identical to the first part of the information stored within the mark - as if the whole story had been copied, then scattered through various parts of Hell, probably as a security measure to prevent loss of information. This is what I managed to uncover.
The Princesses listened with rapt attention as a canvas of chaos magic formed into a sort of enchanted stone with a green symbol glowing menacingly on its surface. Suddenly, a voice more terrifying and deep than anything the Princesses have heard began to speak. The voice was horrible, instilling fear into all Princesses present. Princess Celestia, in the first time, was genuinely afraid. She hasn't felt this powerless even against monsters of ancient times, not even against Lord Tirek.
Then suddenly realization struck her. The supposed owner of this menacing demonic voice was horrified of the very being that set foot in Equestria not an hour ago. As the voice went on, Celestia has come to the conclusion that whatever this monster would demand from her, she could hold no power to deny it. She just hoped that he would be pleased by being given the rule of Equestria to him, and he would spare her ponies. She felt, no, she KNEW that even the Elements of Harmony couldn't contain a being who has literally overpowered a whole universe on his own. Little did she know that the very same being had only one goal in his mind: leave Equestria behind and return to his world as soon as possible.