Until the moment he found himself laying face up on the cobbles of a Canterlot back alley, bleeding out from a bite on his neck, the stallion that had been known as Agent Sadness had thought he was the luckiest stallion alive.
Months ago, during the Agents of Chaos attack known as "Operation Whirlwind", the stallion had been tasked with "seeding" Canterlot with dangerous magical artifacts. He took the task up with great trepidation, knowing it would practically guarantee him being cut off from the rest of the rebellion. Nevertheless, he succeeded in flitting from back alley to back alley, dropping artifacts as he went, then used the last one he held - the "Chameleon Collar" that kept him hidden from sight during the whole effort - to escape the city while the loyalists were doing their counterattack.
He had remained fugitive for several days, ditching the "Chameleon Collar" in the process, and made his way to Cloudsdale where he deliberately surrendered to the local garrison of E.U.P. Guard members. Since the members of the satellite branch didn't have any means to prove his story, they got Princess Celestia's approval to give him to a life sentence in Canterlot Supermax instead of applying the death penalty other Agents of Chaos had suffered. It was in this prison that he met a pegasus stallion that claimed he served a "Queen of the South" and could lead him to that queen if there ever was a chance of escape - an offer that the former Agent Sadness eagerly took him up on.
Discord's release and disruption of Canterlot Supermax provided that opportunity, and the two pegasi stallions escaped into Canterlot's back alleys. It was only there that Agent Sadness found his luck had run out, as before his eyes he saw his partner turn into a "bug-like" black pony and then lunge with fangs at his neck before the ex-Agent of Chaos could respond to the attack.
As the Agent lay on the cobbles, knowing that the situation was too disrupted to realistically hope for rescue as he bled out in the alley, he saw his partner give a mocking "laugh" that sounded more like a snake's hiss, then in a flash of green magic turn into an exact copy of his appearance. Another former Agent of Chaos was now replaced by a Changeling drone, and soon would join forces with another drone infiltrator...
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The main objective of the drone once he had replaced the former Agent Sadness was to reach the cloud elevators that connected the main city of Canterlot with the pegasi "colony" built on permanent cloud banks above the city proper. Although in theory it was possible for a pegasus to fly between both areas without using the elevators, that method would've been a labor-intensive and awkward chore for any pegasus carrying a load or pulling a vehicle, so most pegasi opted to pay a toll for the relaxing ride up and down standing on a cloud instead. The toll, in turn, paid for the laborers to push the cloud up and down the semi-permanent cables strung between the cloud colony and a ground station as well as replace clouds if need be.
Discord's disruption had taken away some of the clouds normally used as part of the elevator, so as the disguised Agent Sadness approached the ground station he could see a line of irritated pegasi backing up as the operators scrambled to try to get the service restarted. The Agent took cover in an alley, looking up and down the line, and near its end spotted the also-disguised Agent Deceit, the filly Cozy Glow just behind him balancing a small bag on her back!
The disguised Agent Sadness gave a low whistle in the direction of the other disguised Agent, and as Agent Deceit looked over in response Agent Sadness sat on haunches and crossed his forelegs in an "X" shape over his chest. Deceit recognized this symbol - a quick way for disguised drones to identify themselves to other potential drones in disguise - and trotted over from his place in the line, Cozy Glow reluctantly following behind him. The pegasus behind Deceit and Cozy Glow in line gave an odd look to the duo as the place in front of them in line opened up, but otherwise did not pay attention to what was going on.
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Agent Deceit and Cozy Glow followed Agent Sadness into retreating further into the alley, now completely out of sight of the line to use the cloud elevators. Cozy Glow was understandably icked out by her surroundings and confused by what was happening, but said nothing as the two disguised stallions engaged in conversation, Deceit speaking first:
"The line to the elevators is way too long, we'll get caught before getting to the head of the line. Got any ideas?"
"Can't we just speedfly directly?"
"Not with little Glowworm having to carry her bag. Besides, I'm pretty sure that there will be a lot of cloud-wrangling going on. We can't afford to be in the wrong place at the wrong time."
"Hmm, we'll have to commandeer a cloud platform then. You know a way how?"
"Yeah, off-duty guard cover. Come on!"
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"Wait! You're not gonna leave me here, are you?" Cozy Glow calls out with a sad face as both disguised stallions start further down into the alley without her.
"Of course not, Glowworm. We'll just be back in a minute." the disguised Agent Deceit calls back in answer, hoping that the filly wouldn't ask a bunch of questions about where the two stallions were able to obtain E.U.P. Guard armor in the span of a minute or so afterward.
"Are you gonna have to hurt ponies? Because that's gonna make everything harder..." Cozy Glow asks again.
"Other ponies will only get hurt if they act stupid. Just like you get hurt if you act stupid, so stay right where your hooves are now and we'll be right back." the disguised father answers again, with both he and the disguised Agent Sadness taking off before Cozy Glow could say anything else...
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The Changeling drone disguised as Agent Deceit had half a mind at that very moment to abandon Cozy Glow completely, but changed his mind when he realized the filly would still be potentially useful in the future. He would continue to mold and warp her mind to suit his needs, or drain love from her as an emergency supply if nothing else.
It was thus that the Changeling infiltration would continue deeper into Equestria, following the "strategic elimination and replacement" policy pioneered by Agent X and endorsed by Queen Chrysalis as a means to set up her own attack on the kingdom. The release of Discord would leave much confusion, incorrect assumptions and mysterious puzzles that paved the way for the Changelings to attack themselves...