Though historians wouldn't officially declare it so for another century, for many that day was the end of the Age of Harmony. Whether what came next was for better or for worse depended on whom you spoke to, but no matter what the opinion things had changed, and they changed from the top.

After recovering from the loss of both property and equine life, Princess Celestia and Princess Luna immediately reorganized the Royal Guard to be more stringent in its selection of candidates, as well as retraining the current guards to be more effective. Twilight Sparkle and her student Starlight Glimmer were called in to help rapidly increase the technology the Guard could have access to, starting with attempting to replicate the weapon the Doom Slayer had left behind. In addition, Stalwart and his guards who had witnessed a bit of how the Doom Slayer fought firsthoof were the first to be trained in the use of this new technology, as well as training new recruits in the tactics and weapons to emulate the violent destroyer.

News of Equestria building up its military had initially been dismissed or ignored by the other nations, thinking it to be rumors or exaggerations. It wasn't until about twenty years later when a Diamond Dog by the name of Morton the Merciless had began taking over and recruiting the other packs under his banner that the world learned exactly what the Equestrians had been up to. No one doubted that Morton's forces would have been defeated. Even with his massive army nearing half a million soldiers, he was not a military strategist. His only strategy was to attack relentlessly and in overwhelming numbers. What the other nations did not expect, however, was for his forces to be completely routed and for Morton himself to be killed in combat in just three days.

The other nations finally noticed the changes that were made and were terrified of the power and ruthlessness of the new Equestria, afraid of conquest. Despite the reassurances from the Princesses almost all the other nations began to ally together in fear over this new potential enemy, but no one wanted to strike the first blow. The Princesses' constant assurances that they would only deploy their forces in retaliation had little effect on easing tensions, but things finally began to ease when Equestria made a concerted effort to try and rebuild their neighbors in Griffonstone, attempting to recover the relics of their past and even helping to finally have a ruling government again. Though it would take another century for it to happen, international relations finally returned to normal. The world finally had begun to understand what Equestria was now: It has always been a powerful beast, but it had not grown more vicious. It had simply learned how to sharpen its claws.

The Princesses themselves however were more interested in the opinion of someone else, someone who was no longer there. But they would finally get their answer three hundred years after that fateful day.

The Doom Slayer appeared in a flash of red light, taking a moment to recover himself before observing his surroundings. The sky was red, the obvious sign of Hell's influence, but the structures were not what he was used to. Instead of the industrial greys of the U.A.C. or the browns of Hell, this seemed a mixture of white and grey. Ancient marble had been updated with technology, with care being done to try and preserve what was there originally while still keeping everything functional. If it wasn't for the dark sky he would've almost said it looked… colorful. He seemed to vaguely remember another place he visited being like that as well…

His attention was caught by the sound of gunfire, some of it off in the distance, some of it much closer. He cocked his combat shotgun, ready to take on whatever was ahead, only for two creatures to round the corner to face him. Both they and he stopped to take in what they were seeing.

The Doom Slayer saw a pair of equines, which he could only tell from their shape, holding some kinds of rifles out in front of them, covered head to… hoof and armor that looked remarkably like his.

"Oh my… Is that really-?" one of them asked, completely shocked by what he was seeing.

"I… I don't believe it. It's really him…" the other muttered before touching a hoof to the side of his helmet. "This is Private First Class Windrunner. I'm with Private First Class Hardstone on Emerald Ave. We have a… a code 1993."

"... Say again Windrunner, you have a what?" came a voice on the radio.

"Code 1993."

"... You're sure?"

"No doubt about it."

"... Acknowledged, stay with the target. The Princesses will be with you shortly."

"Roger."

It was only fifteen seconds later that two large ponies appeared in a flash of light, both dressed in similar armor to him but with small differences, like how one had gold linings and the other blue.

"My goodness, it is you." Said the one with the gold linings as she removed her helmet, as did the blue one. "Do you remember us? It's been so long, centuries for us. I have no idea how long it's been for you."

The Doom Slayer had to rack his memories for a moment, but he did eventually remember fist bumping a large white pony a long time ago. He lowered his weapon in acknowledgement.

"You came and helped us back then, even if that wasn't your intention. We… We learned from you, even if you didn't intend to teach us. Equestria had alway been a place of harmony and tolerance, where we attempted to help those that would oppose us, attempting to appeal to their better nature. We still do as a whole, but… we've also had to face the truth. A truth you showed us." The other pony levitated a finely polished wooden box over to him. "We learned that for some creatures there is no better nature, that there isn't anything that can be redeemed. Some beings are just monsters pure and simple. And monsters can't be reasoned with." The pony opened the box, revealing a familiar weapon he had lost long ago. "Monsters just need to die."

The Doomslayer hesitated a moment before picking up the super shotgun. It was well maintained, better than he had ever managed to keep it, like it was treated as some kind of jewel. He almost felt bad about the blood and gunpowder residue he was going to get all over it. He opened it up and inserted two shells just as a group of Imps rounded the corner. They all froze when they saw him.

"We attempted to learn ourselves what needed to be done. But we can only do so much on our own," the pony said, her voice turning harsh as she and the other pony put on their helmets, raising their weapons. The white pony held up what looked like a heavy assault rifle with micro missile attachment, while the blue one had what looked to be a gauss cannon with precision bolt. "Do you have any advice for us, Slayer?" she asked.

For the first time in a long time, longer than he could remember, the Doom Slayer was… happy. Not happy like the satisfaction of killing a demon, a happiness he had not felt since before his crusade against the Legions of Doom. He had taught an entire people to fend for themselves, to make themselves strong against the hordes of darkness that would seek to wipe them out. It made his heart swell with pride… which quickly mixed with rage and sadistic glee. He turned to face the horde of demons as he smiled, levelling his weapon and saying the words that would become the battlecry of Equestria for ages to come.

"RIP. AND. TEAR."

I hope you all enjoyed. Any feedback is welcome. I've got one more story to post on here, but it isn't finished so you can look forwards to enjoying that one for a while. Another MLP crossover, this time with a dark, but not so gritty Playstation-era IP. Hope you look forward to it.