I don't know much about My Little Pony and as a point I don't include much about it in the fic. If you are a brony, you will probably be highly offended by the following pony slaughter, literally and by the fact that I know very little about MLP. Also I don't own anything. You have been warned…
Hecarim was not a happy undead centaur. He was just about to lead a regiment of spirit cavalry from the Shadow Isles to conquer the mainland, when this bright beam of light shot down, and lifted him up only to drop him into some unknown forest. And I was just about to kill everything in Valoran too, he thought.
His depression was interrupted when a tiny yellow creature came out of the undergrowth. It looked like a pony, but with tiny wings. It had a pale yellow coat and a pink mane that looked more like some weird hairstyle. It looked up at him with freakishly large, tear-filled eyes.
"Wh-who are you?" She asked, the high pitch of her voice betraying her gender.
"I am your death." Was the simple reply, before Hecarim swung his giant glaive. The blade passed cleanly through the pony-thing's neck. A second after both the body and the head fell to the ground; a ghostly pale form of the pony floated a foot above the corpse.
"Hmph. Not very strong, but if the rest of this world's inhabitants are like her, that won't be a problem." With that statement to no one, Hecarim trotted through the forest to find his next victim. The pony ghost shuddered, and then followed its master in death.
Three months later
"Ah, Hecarim. The Shadow Isles were wondering where you ended up." A giant suit of armor stomped up to the undead centaur. "Quite disappointing really. We were all ready to conquer Valoran, and then you disappeared. At least you seem to have had some fun here."
Hecarim grunted in annoyance as he looked over the land once known as Equestria. The land was decaying, dying without its inhabitants to maintain it. The ground was a dull gray, ghostly fires blighting the land and killing all life on it. Pale, four-legged forms wandered aimlessly through the wasteland, a tribute to the passage of the Shadow of War. While the slaughter was fun (and proved that one could not spell 'slaughter' without 'laughter'), the ponies were weak, and offered no resistance.
"Yes, Mordekaiser, for a time, they were fun to kill. But then it became a chore, there was no challenge except in the finding of the spineless cowards. But I have gathered souls to be used in our army, as well as having learned something."
"And what did you learn from such pitiful beings?" Mordekaiser questioned the dead centaur.
Hecarim turned to Mordekaiser with a triumphant gleam in the fires that were his eyes, and loudly proclaimed:
"SUFFERING IS MAGIC!"
Had this idea for a while, and it was too great not to write a oneshot.