From "Analysis Of Popular Legends And Mythology", By Starswirl The Bearded.
An excerpt from Chapter Twenty-Seven, The Hero Who Vanquished The Great Evil.
I sincerely question whether this tale is a legend or myth, but instead may deal with something that truly did happen at one point in the distant past. Before I begin, let me give you a scenario that may help better explain why than a simple explanation.
We discussed, in chapter five, the Boojum and how such a beast could not exist as anything other than a cautionary tale to keep colts and fillies from misbehaving. Let us say, theoretically, that one pony told you that there was a cave nearby that housed such a creature. You would likely consider such a fellow a fool. Then, however, you are told by ten ponies that they have indeed seen the boojum, and that it indeed lies within that cave, and had even spirited away a colt or filly last year, never to be seen again. You'd think that they were likely mad or gullible, and suspect that something else was more likely to have done harm to that young pony. Then, however, if you speak with every pony in the village, you are told that each and every one of the residents have seen the creature with their own eyes, eight hundred ponies in all, and they can give you a detailed description of the boojum, its habits, a list going back three centuries detailing each of its victims, and can even tell you that the creature walked through the middle of town yesterday at high noon, where everypony could see it clearly, before going back to its cave.
Now, in that situation, a rational pony could come up with a dozen excuses for what could have happened that would explain away this "Boojum", but a reasonable pony would also keep well away from that cave if he knew what was good for him. An especially reasonable pony would stay away from the cave after just ten ponies had corroborated the tale. Just because the boojum isn't real doesn't mean that SOMETHING isn't in that cave, and there are many creatures out there that can kill an unwary pony more dead than any fictional boojum ever could.
Returning to the original point of this chapter, I'll now explain why I feel this may indeed have relation to an actual event in the past. The reason is simple: Everyone tells this same story. Every culture, no matter how remote, tells the story largely the same way. From every corner of the globe, no matter how distant, the story is told and told largely the same way. Only a few details are different, and many of those are largely trivial. Later in this chapter, I'll go into detail regarding those varied and admittedly minute differences. For now, however, I shall describe how all of them are alike.
First, there is always a hero. No two species agree on his race, and he is always a member of the race of the storyteller, but the hero's existence is always a constant, and he is always male. He is also never alone: The hero has a magnetic personality, and even if he has a few odd quirks here and there, and is never described as physically attractive, he is so inherently charismatic that he draws others to his cause without much difficulty. And while the his comrades vary in description, they likewise share qualities we shall go into detail about later in the chapter. While other qualities are discussed and vary from place to place, these qualities are always universal.
Second, there is always a "Great Evil". While some stories vary wildly in description, the fact that all the tales describe it as a powerful shape-shifting entity in addition to all the other abilities it possesses, this means that the entity's shape was most likely always in a state of flux, with no set form, explaining that variation in appearance. This matches the appearance of the "Evils" that ravaged the land, until the alicorns put them down. This is appropriate, since these creatures are believed to owe their existence to the very vanquishing of that great evil in the first place.
Third, there is always a magical sword, and a quest to obtain it. Why a sword? Who can say? Perhaps this sword was specially forged for the purpose of slaying the Great Evil. Here, however, the details are almost always universal, to the point that I have only been able to find one variation on the theme, which I will describe to you now.
According to the Goats, after the great hero reached the end of the quest for the weapon to defeat the Great Evil, he was presented with two swords. The first was the weapon that is described in every tale, a blade known as The Sword of Destruction, a powerful blade that could cleave mountains and slay even the mightiest of beasts in a single blow. The second blade is not described in detail, save that it was a strange sword that the hero could not understand, and named only The Sword Of Reason, a contradiction in terms if ever there was one. The hero was told that while the first blade could strike the Great Evil down, it would not truly destroy it, and it would eventually return in one form or another. Worse, slaying the Great Evil would cost the hero his life if he used this sword. The second blade, as strange and incomprehensible as it was, could destroy the Great Evil forever, and without the cost of the hero's life, but only if the hero could truly understand the weapon and utilize its full power. If he could not, the Great Evil would surely end his life in a heartbeat, and there would be no one left to stop it.
The goats say that the hero took The Sword Of Destruction, since he could not comprehend this "Sword Of Reason". Perhaps, if he could have, the world would not have undergone the trials and tribulations that it has since, but he may have been wiser than we think, given that he went with a guaranteed victory over a risky gamble, even if it did cost him his life.
From here, the story ceases its deviation, and continues the same as it does for all races: The hero and his band journeyed to face the Great Evil, and after a long and terrible battle, the hero used The Sword Of Destruction to deal the finishing blow, at the cost of his life. From there, the hero's companions return home to tell the tale to the varied peoples of the world. Et cetra, et cetra.
I sincerely wish that there was more information available regarding this "Sword Of Reason". After all, even with the Evils defeated, we will never be certain if the Great Evil, in one form or another, may not someday return. And even if it does not, there may come a day when an evil may appear that is so terrible that it will require that strange weapon's power to defeat it...