Princess Celestia was a busy mare. Thankfully, her day was just about over, and she'd be able to turn over duties to her sister for the night. Meanwhile, she'd be in bed, getting some well deserved rest.
This was not to be, however, as a familiar green wisp of smoke flew into the window, and materialized into a scroll. Sighing, Celestia unrolled the parchment and read through it. However, instead of her student-turned-fellow-princess's usual writing, this was much more rushed and brief.
Princess Celestia,
Sorry to bother you, but come to Ponyville quick. Humans, zombie things, and something called "dosh".
-Twilight
Perplexed, Celestia fetched her personal telescope and looked at the town not too far from Canterlot. There, she saw six unidentifiable blobs who seemed to be fighting off a large number of other unidentified blobs.
Celestia shook her head. Nothing could ever be normal, could it?
"Hold on a tic, I'm reloading!"
"I've seen scarier five-year-olds!"
"I'm hit! Bloody well do something!"
"You muppet!"
The ponies watched as the humans fought off yet another onslaught of those monster things. Alongside all the other things that were attacking them, now there were relatively small, shriveled ones bound in straightjackets, who attacked by screaming very loudly. They were really the only ones who could hurt the ponies, though only due to sheer volume. They were taken out fairly quickly, anyway.
Other members of the six humans died in battle, and again they'd just be good as new once the "wave" was over and they all ran to one of the metal doors. While Twilight and her friends were watching them cautiously, they had lost a lot of interest in the proceedings. Fluttershy woke back up during one of the breaks, and after the situation was explained to her, she decided simply to hole up in her house for the duration of whatever this was.
While Twilight was watching the humans once again run out of her library, Spike came running. This shook her out of her mild boredom. "Spike? What's wrong?"
Spike wheezed. "The princess... Sent back a... letter..." He held up a scroll, which Twilight quickly seized in her magic. Opening it, she read the reply.
Princess Twilight,
I have seen what is going on, and I will be there shortly to receive details from you.
-Celestia
As she rolled the letter back up, Twilight turned just in time to see one of the humans decapitating a red creature with his sword, resulting in the slow motion that was almost routine, now.
The human moved on, killing any creature in its path while, she assumed, making his way back to the others.
Next to her, Rainbow yawned. "Not that something like this is boring, but... it's getting kinda boring, Twilight."
"I know, Dash, but I don't know what these creatures are doing here, or if they're any danger to ponies in any way. It's my duty as a princess to find out these things, after all."
"Well, while I cannot condone such barbaric activities," Rarity said, "but perhaps those 'humans' are... protecting us, somehow?"
"Now that's just silly," Applejack replied. "If none of 'em can touch us, why would they need to protect us?"
"I don't know, girls," Twilight admitted. "The longer this goes on, the more I question just what is happening."
Once again, the humans ran by, asking for money, if Twilight was right in her assumptions. If her guessing was correct, this was the fifth "wave" that had passed. She didn't know if there was an end to this, or if it just went on until the six humans all died... Er, all died at once.
Soon, however, Twilight noticed something in the sky approaching Ponyville. For a second, she thought it was yet another creature bent on mindlessly shuffling towards the six humans, but instead, she saw the unmistakeable flowing mane.
Twilight almost bowed when Celestia landed, before realizing she didn't really have to anymore. Her friends, who were nearly asleep, didn't, either. "Princess Celestia! I'm so glad you could make it."
"As am I, Twilight." Celestia looked around, looking suspiciously at a nearby bloodstain on the ground. "So, what exactly is going on?"
"I'm not entirely sure, Princess." Twilight pointed further into town. "About twenty minutes ago, metal doors were found on seemingly random buildings throughout town, including my library, and then right after that, six things that are apparently called humans showed up, but then they were attacked by a whole bunch of other weird and rather horrifying things, and-" Twilight halted her explanation. "Look, here they come now!"
Sure enough, the six humans were returning from wherever they were. They took out the devices they usually took out after breaks in the fighting, and one of them even brought out a sword. Taking the initiative, Celestia walked forward towards them. "Hello, may I ask who you are and what you're doing here?"
The six humans all looked at each other, before one of them, wearing a suit and gas mask, walked forward. "Smeghead."
"...oh." The human then walked forward, passing right through Celestia. "Oh..."
"That's another thing," Twilight continued, "they just pass right through ponies, them and the things attacking them, as well."
"Well..." Celestia paused. "I can't say I have any clue about what is going on." Suddenly, several creatures could be heard moaning and groaning nearby. "What was that?"
"The creatures I mentioned," Twilight said, plainly. "They're scary, but they also can't touch ponies, and don't even seem to notice us."
While scanning the oncoming horde, which was already being dispatched by the humans, making Celestia flinch at the blood and flying limbs, Twilight noticed another new creature advancing. This one wore what looked like basic medical scrubs, but one arm was replaced by a revving machine with a blade of some sort.
"Please tell me that's not a chainsaw he's carrying," one of the humans commented. Notably, unlike all the other monstrosities, the humans didn't immediately attack the new one. Instead, once all the nearby ones were dead, the sword-wielding human ran forward and began chopping at the "doctor" creature, causing it to continuously flinch until eventually its head was lopped off.
"They seem to attack in waves," Twilight went on with her explanation, "and between these waves one of the metal doors opens and the six of them head inside, where they just stand there and throw money at each other sometimes. There's also a seventh human inside, no matter where it is, but she never moves or leaves the building."
Celestia was too puzzled to comment or question anything. Instead, she watched as the humans kept fighting, one of them throwing a device at their feet. However, instead of the explosion Twilight was fearing, a green cloud appeared just above the ground, which seemed to hurt the creatures while somehow healing the human.
Once all of the monsters were dead, the humans once again walked off, Celestia and Twilight in tow, leaving a sleepy Applejack to drag her already-sleeping friends to somewhere that isn't the middle of the street.
At the town hall, where the town's ponies had set up a picnic of sorts while waiting, Celestia and Twilight followed the humans through the metal door. The ponies bowed upon realizing Celestia was there, but she ignored it.
Celestia watched as the humans engaged in whatever they did to get new devices out of thin air. "So, Twilight, what do you think? Is there a threat to ponies, here?"
Twilight hesitated. "I... I don't know. I mean, they can't touch or interact with us in any way, although the humans at least seem able to see us, but..."
Celestia nodded at her fellow princess's unspoken worry. "We will continue watching them, just to be sure."
Suddenly, both her and Twilight found themselves outside of the town hall, the metal door closing in front of them. Confused Celestia looked around.
Twilight smiled sheepishly. "Oh yeah, I should've mentioned that..."
Elsewhere in the town, one of the humans remarked, "You couldn't scare my mum!"