Agent 21 did not fear the trip on hooves to Canterlot. After his artful escape from Ponyville, he rested for the night in a clump of bushes just outside the patrol range of any Royal Guards along the Ponyville-Canterlot road, then at first light of Celestia's sun started up the path to Canterlot. 21's objective was to make it to the main gate of the city by sundown, then proceed from there to the final objective Agent Snake had given him...
He didn't fear interference from the patrollers. The road, which roughly paralleled the train route between Canterlot and Ponyville and ran atop a natural ridge up the mountainside of Canterlot, was lightly patrolled by squad of guards, nominally independent of the command in Canterlot but getting supplied by them, based out of a pair of customs checkpoints placed along the route. The guards lacked proper equipment to do much more than patrol the road and perform customs checks, and its membership was generally composed of guards out of favor with the current upper-level administration but unable to be discharged or transferred elsewhere.
Agent 21 knew he could easily dupe these disaffected and unempowered Guards with fake identifications, among other things. With the Canterlot-Ponyville ground route being a convenient "dumping ground" for the politically unfavored and too much confidence in the direct defenses of Canterlot, this part of the Guard made a notable weak link later exploited by the Changelings in setting up their invasion of Canterlot, and now would allow another attack of the Agents of Chaos...
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EARLY THE FOLLOWING EVENING...
"You wanted to see me, sir?" Guard Upper Class Buck Withers asks as he stands in front of the desk of Lieutenant Captain Shining Armor.
"Yes, got some bad news for you." Shining Armor replies somberly, using magenta-colored magic to levitate up a sheet of parchment from the desk. Buck frowns as the parchment is levitated in front of him, then a combination of shock and dismay spreads across his face as he begins reading the parchment.
"She never regained consciousness, Buck. Shortly after noon today, her heart stopped and the medics couldn't restart it. She's gone, Buck, I'm sorry." Shining continues as he lets the parchment drop back onto the desk.
Shining is silent as Buck stares down in dismayed grief at the death certificate with his mother's name on it laying on the desk - he remains silent as he could see Buck trying not to break down in front of him. Nevertheless, a few squeezed-out tears manage to trickle their way down the face anyways.
"Request... request one week's leave, as per... per regulations." Buck finally manages to force out.
"Granted, but you'll still have to work tonight." Shining replies, and when Buck's questioning expression falls on him, he continues on with "I don't have anypony to replace you because of the staff shortage. I even asked the Captain to hold a pony over from the afternoon shift, but he said that would be against regulations. Sorry Buck, there's nothing else I could do to help."
Buck gives a disbelieving glare at Shining's meek expression as the former gently grabs the parchment off the table with his mouth and puts it in the duty bag slung at his side. The disbelief continues, which seems to compel Shining to speak again: "For tonight, I've put you to guarding the main gate of the city. Should be quiet there, you'll have plenty of time alone with your thoughts."
Buck's continued disbelief in his expression is obvious indication that Shining's assignment has not proven the sincerity of his words, but at least it keeps any accusations caged in his mouth. He just leaves the office, not even waiting for any formal dismissal orders. As soon as Buck was out of the office, Shining blows out a loud breath.
"Secretary?" Shining asks, turning to the grey-coated unicorn quietly scratching quill pen on parchment on a small desk to the side of Shining's own.
"Yes, sir?" the secretary replies, not even looking up from his work.
"Clear my schedule for tonight. I've got a sense I'm going to need it that way. And have Lieutenant Skyver please summoned to my office." Shining answers.
"Yes sir."
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Guarding the main gate into the city was not a glamorous job. With the drawbridge over the small but fast stream running in front of the gate raised and the gate itself closed and locked, the guards there were to prevent admittance until Celestia's sun rose in the sky again. Besides a pair of guards stationed between the gate and drawbridge, there were two more pegasi posted atop the gate, to prevent anypony from simply trying to fly over the wall. However, with very little for them to do, they had an annoying tendency to doze off, leaving only the two ponies on the ground as a real defense.
Buck was at his place, alone. He sometimes glared over in annoyance across the road, where his partner in the duty was supposed to be - he had not shown up as he was supposed to, and with Buck cynically assuming the pegasi atop were already dozing off he feared he was alone. He also thought his CO had a lot of nerve putting him in this situation.
Buck's thoughts were flitting from place to place again. He thought none of his foalhood life was still around - his friends and extended relatives having disowned him for what his father did helping him, the same father betrayed and killed without so much of an opportunity for a goodbye, and now his mother thrown into a nightmare she had not managed to survive. The only relationship he still had from before he joined the Guard was Lemony Gems, and she obviously couldn't be here now to help him.
He hated it all. And he hated Shining Armor for being the ultimate reason he was in it. But before he could slip back into the old pattern of hatred, his conscience rose up and gave a metaphorical slap to the face. It reminded him his life had been destroyed not by the virtuous brother of Twilight Sparkle but by the scheming and depraved ponies who wanted him gone, for whatever their twisted reasons. It was them that he should be hating, not his CO.
Very quickly Buck was snapped out of his thoughts by the sound of galloping hooves. In the faint moonlight he could definitely tell it was not somepony in royal employ. Before Buck can even call out, the cloaked pony makes a mighty leap over the stream to land beside Buck.
"State your identity and business, intruder." Buck firmly states, leveling his pike in front of the intruder.
"I am Agent 21 of the Agents of Chaos." the cloaked intruder calmly states, turning to face Buck with red eyes glowing under the cloak and pushing Buck's pike aside.
"You freely admit you are an enemy and a traitor?" Buck challenges.
"Yes, because for you it does not matter. Your end is my business here." was the Agent's smooth, sinister reply.
"And what in Equestria is that supposed to mean?" Buck continues to challenge.
The Agent lets out a low chuckle (of bemusement, it seemed to Buck) before pulling back the hood of his cloak.
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Buck received another shock at the face that was revealed underneath, and how much it seemed to have changed from the last time he saw it. The eyes glowed an evil red - an effect Twilight Sparkle would later see on a corrupted Trixie wearing the Alicorn Amulet - and the voice was what we would call "demonic", not like the one Buck remembered him having at all. Buck later remembered seeing a red-glowing outline of something attached to the neck, but couldn't make out what that something was.
"They threw the book at you, Flank Thrasher! You were put away for a long, long time! How did you get out of prison?" Buck exclaims, his face clearly showing disbelief at what his eyes were seeing! The reveal had succeeded in getting his guard down, and the Agent would go in with the strike...
The Agent begins pacing back and forth, always careful to keep Buck's pike shoved aside, speaking as he paces: "It doesn't matter how I got out. You see this, Buck? It's a power, a power growing within me. Just like the power of chaos is growing. Are you frightened yet?"
"No!" Buck exclaims in defiance.
"You should be. Oh, but you should be!" the Agent continues on, pacing continuing without even the slightest pause - deliberately done to continue unnerving Buck. "The power of chaos will never be stopped. Harmony will end, the chaotic one will rise, and Equestria will fall! And in that faith, I have the power to destroy you!"
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The Agent leaps for Buck's throat, knocking away the pike and causing it to roll into the stream nearby. The two earth pony stallions go down rolling, Buck now worried of being knocked into the stream and over the nearby falls to death or grievous injury.
The noise of the struggle is supposed to prompt the pegasi above the gate to come down to help Buck, but there is no movement from them - out cold sleeping, as usual. Whirling in the low light, Buck realizes he is without help or his weapon and has to improvise a defense.
Both stallions are dizzy and Agent 21 rises to four hooves in order to reorient himself. This gives Buck opportunity to work off his loosened helmet and hurl it at the Agent. The helmet deflects off at an awkward angle and goes into the stream, quickly to be lost over the falls. Buck tries to get onto his hooves, but the Agent charges like a bull and pins him on his back, forehooves going on his neck.
"It didn't need to be this way, Buck! We could've been in this together, just like old times! But you betrayed us, and now you must die Buck Withers!" the Agent exclaims, spitting in Buck's face and with murderous intent in the expression.
Buck, deprived of air, wriggles in vain to get free of the choking pin. The world around him starts to grow blurry and even darker, but before he can pass out completely a white pony-like shape in a glowing purple bubble passes the corner of his vision and the choking pin comes off!
Gasping for air and disoriented, Buck never sees what happens next, but he sees a flash of purple, more sounds of struggle and finally hears his opponent shouting as if he had fallen down the cliffside parallel to the waterfall of the stream. His vision is still swimming several seconds later when a blur of white, blue and purple moves into his sightline and asks if he's alright.
It takes several more seconds of coughing and trying to focus before Buck can respond with a nod. He hears the voice of his immediate superior in the background, apparently berating the pegasi that had been asleep on the gate. A purple glow surrounds him, giving him an assisting lift back on all four hooves. It is only then that he recognizes that it was Shining Armor's intervention had saved his life!
"Thank... thank you, sir." was all the breathless Buck could say at the moment.
"Had a hunch you might get attacked out here. Turns out I was right, now I'll have to wait on what Lieutenant Skyver finds out from the pegasi atop the gate." Shining Armor replies, looking in the direction where the Agent of Chaos had been punted over the edge.
"Sir, what about...?" Buck tries to ask as he continues trying to get in air to his aching lungs.
"Your partner? Another Agent - broke rank after being dismissed to his post, currently AWOL somewhere in the city. Lieutenant Vesper and his bat ponies are looking for him now." Shining Armor explains.
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"Sir?" a voice sounds from above. Buck and Shining Armor look up to see Lieutenant Skyver peeking down from the posts atop the gate.
"Ah, go ahead Lieutenant." Shining Armor replies while continuing to look up.
"Just as you thought, napping on duty. I recommend court martial for the both of them!" Skyver shouts back down, referring to the two pegasi that were supposed to be watching atop the gate.
"Very good, have them confined to quarters and make sure to leave all the paperwork on my desk. I'll attach the motion for court martial on my incident report." Shining replies.
"Understood. Anything else, sir?"
"Have Lieutenants Aldebaran and Stratocumulus send two ponies each from their respective squads to cover the gate here. I'll stay here until you can get those ponies to relieve us."
"Yes, sir!" Skyver replies with a salute before flying off, slowly tailing the two loafing pegasi to make sure they actually went back to the barracks as they should.
When Skyver is gone, Shining looks over at Buck and blows out a big sigh. The exchange of looks shows both of them knew how close it had come to another Agents of Chaos victory here, only thwarted by Shining's intuition and careful planning.
"Sir, should I...?" Buck asks Shining, pointing a forehoof at where he was supposed to be stationed.
"Yes, go back to your post. We'll worry about your equipment later, nothing we can do about it until relief arrives." Shining comments, taking the spot across the pathway for the watch.
Buck Withers shuffles back to his post, but can't help but spare a glance over the precipice, wondering what had happened to his old friend that had gone over to "the dark side". He also wondered about the "growing power of chaos" - what exactly did it mean? Was there yet another plan by them in place, somehow and some way? Was he supposed to die as part of that plan? What would happen to him anyway? So many questions, and yet no answers for any of them...