So, here's the story, man.

There's evil, there's super-evil, and then there's Grogar. Grogar was Evil with a capital E. One of the big prime Evils of the world. These days, Equestria is really peaceful, and there's a reason for that: The Evils were all killed. Not sealed, but flat out slain. Celestia and Luna saw to that. And there's a reason for that, man.

See, most 'evil' is only evil 'from a certain point of view'. Stallion steals bread, evil. But a poor stallion takes a loaf of bread from a fat and wealthy merchant to feed his starving family? Not evil. A lot of the things we see as evil are just a matter of perspective. A lack of context. When most guys do 'Evil', they don't consider it 'Evil', they have some sort of justification to it.

With guys like Grogar? The Evils? Completely different thing.

In The Beginning, when the Haz-Bo-Ro and the Faust wove the world together, there was a thing that was required. This thing was called Evil. The sum total of all Evil that was meant to be in this world was sealed away, so that only good could florish in the world of Equestria while the creators were at work. However, there was an argument between the Haz-Bo-Ro and the Faust, regarding things that our puny mortal minds cannot comprehend. In the end, The Faust and the Haz-Bo-Ro parted ways, and somehow, during this argument, the seal of Evil was accidentally broken, and darkness spread across the land.

At first, it was just one evil, an ultimate villain out to conquer the world, until a hero and his plucky band... yada yada, struck down the one true evil, you know this story, heard it a thousand times a thousand different ways, so I won't bore you with it. What mattered was the result. Evil was shattered into pieces. These pieces became their own distinct entities. They became... The Evils. And of all the evils, Grogar was the smallest and weakest.

So how did he become the most feared and the most terrible of all the Evils? Simple, man. Because he was the smallest and weakest, he had the greatest incentive to do something that the other Evils did not: Learn. The other Evils were massive and terrible things that could level mountains with ease. As such, they had no reason to learn anything new. Up until the rise of the alicorns, the Evils had nothing to fear, and by the time the Evils realized that the alicorns were capable of destroying them, it was far too late: Celestia and Luna went through the Evils the way a scythe goes through a field of wheat.

But Grogar, being weak, learned stealth and cunning. As such, he hid himself away in a small domain he created for himself, partly of this world, and partly hidden within the realm of shadows, that he named Tambelon. There he could gather power, while remaining undetected to the world at large. Grogar learned the value of having subordinates, and as such, he gathered the Trogs, misbegotten creatures that had no master, and made them his servants. He learned of treachery and deceit, and turned the court jester, Bray, against the princesses, making the donkey into his subordinate. He learned the value of theft, and stole the powers of the fallen Evils, and took them into himself. Grogar learned the value of patience, and spent decades gathering power. Grogar learned the value of wisdom, and spent that time learning all the spells of the world, and invented thousands of new ones as well. For nearly a century, Grogar prepared. And learned.

When Grogar finally appeared, he marched forth from Tambelon with an army of monsters at his back, and went unopposed for many weeks, as the princesses were ensorcelled into a death-like sleep from a vile enchantment that Grogar, working through his agent, Bray, placed upon the sisters. Had he not underestimated their strength, or the power of the Equestrian wizards that eventually broke the spell, then they might have been sleeping still. In the end, it very nearly did not matter: Grogar's army had conquered nearly half of Equestria before the princesses awoke, and had they slept even a week longer, then the final Evil would have won.

Instead, the princesses woke, filled with fury, and flew to engage the dreaded overlord, intent upon laying him low. You should have seen it, man. It was nuts. Fireballs, lightning bolts, the whole deal. The kind of battle that can't be described with words. In the opening moments of the battle, nine-tenths of Grogar's army was wiped away in an instant...

...But then Grogar took the field.

Grogar had spent decades gathering power, learning everything there was to know. So, by the time that he began his march of conquest, the weakest of all the Evils had become a force more powerful than The First Evil itself. The alicorn sisters found themselves completely outmatched. Every spell they cast was instantly countered. Every strategy they hatched was immediately recognized and thwarted. Every trick they tried to pull was shot down, almost before it could even be thought of. Grogar had their number, man, and it seemed that the two princesses would be completely destroyed.

But then they got lucky. The kind of incredible, impossible luck that comes about only once every thousand years.

You see, Grogar, being the weakest of the Evils, had a weak body. Well, weak for an Evil. He could contain power far greater than a mere mortal pony, but he'd barely be able to handle the level of power one might find in an alicorn. So, in order to be able to possess the power he did, he had to place it within an object. That object was a bell he wore around his neck.

In the final moments of the battle, just as Grogar was about to finish the princesses off, once and for all, Celestia fired off a last, desperate bolt of energy. This bolt was so weak that, even without all of his power, Grogar would have been able to shrug it off. However, the bolt just happened to hit the bell, and hit it in just the right place, with just the right amount of force, and...

The bell cracked. And then exploded. It was a massive blast, and had Luna not summoned a shield to protect herself and her sister, then the alicorns themselves would have been destroyed by it.

Grogar was slain by the power of the blast. Slain, but not destroyed. Bray, along with the remnants of Grogar's army, reclaimed their master's body, and took it back to Tambelon. The sisters, when they recovered, tried to enter the realm, and put an end to things once and for all, but found his domain impossible to approach. Grogar had placed enchantments upon his dread domain in order to protect it, and the spells placed at the height of Grogar's power were far beyond the power of the two sisters.

So instead, they used their magic to send the entire place away, fully into the realm of shadows. It broke their hearts to do so: Grogar's army had taken thousands of ponies as slaves, and by sealing them away, it meant that those slaves were beyond the reach of rescue, by the sisters and by anypony else. However, what choice did they have? Without that spell, we'd be at risk of invasion by the Trogs every hour of every day, and we'd have no way of stopping them, since they could just retreat to their domain whenever we sent an army after them. The princesses did what they could to keep Equestria safe, like they always do.

However, there is a fear that the princesses possess, a fear greater than any other: An Evil may be slain, but unless its body is completely destroyed, it may eventually come back. Worse, Grogar had many titles, and chief among them was Necromancer. Until and unless his body is completely destroyed, we will not see the end of him. Grogar, first and last, strongest and weakest, smallest and greatest, may one day return to wreak havoc untold upon the mortal world.

At least, that's what the shrooms tell me, man. But what do they know? They tell me stuff about pink elephants, and about ponies walking on the moon, and other wicked crazy stuff like that. You sure you don't want one? They will open your mind, man... that, or make you to go crazy. Or both. It's really hard to tell the difference.