The sun shines brightly through the windows of the Canterlot castle as Celestia trots and talks alongside an armored pony through the corridors. An alicorn, clad in silver armor where only his yellow mane and tail flow from under the decorated plate armor. Abaddon, once a heroic unicorn Captain of the Royal Guard so long ago, who was deceived into treachery against the Princesses he swore his life to. Casting away the identity of who he once was in turn for great powers, the now-alicorn turned his blade against his former rulers. Locked away in a nightmare after a failed rebellion, he has finally been returned to Equestria and given the chance to renew his service to Equestria, and mend the wrongdoings of his actions.
"It has been far too long Princess. I have hoped all these years you would accept me back in your wings. Now that you have, I cannot wait to continue my service as part of the Royal Guard." He says as they approach the Guard barracks, seemingly happy to be back. "Tell me, are they still the fierce, unfaltering warriors I had intended to mold them into, so long ago?"
Celestia gives a smile, but beneath it hides worry and uncertainty. "I am glad you are Abaddon, but..." She stops at the twin doors that lead into the barracks. As she turns to face him, her smile fades and her tone shifts into an attempt to hold back what she may say. "I hope you understand, so much has changed since you were gone. We have found other ways to defend ourselves, and with new Princesses looking over Equestria we have nothing to fear."
Abaddon looks in confusion. "What do you mean? Are they not the mighty defenders I hoped they would still be?"
With a gulp, the Princess continues. "They are still the best to me, but we are in a more relaxing time now. The Guards work as hard as anypony else, but they have, well, 'treatments' they go to."
"Relaxing? Treatments?! A Guard does not know these words! The only rest a Guard receives is either the bed of his barracks, or the eternal slumber of death! I put great efforts to turn your once-ornamental excuses for Guards into the best warriors Equestria could count on to crush our foes!" Snorting as his face reddened under his helm, Abaddon already began marching past Celestia, approaching the doors of the barracks, ready to smash them open himself. "And I refuse to believe they are no longer what they should properly be!"
"Abaddon, please understand!" Celestia let out a plea of reason as she approaches him, trying to calm the ancient warrior before he acts out anymore in his anger.
"Understand what, Princess? Is the Guard not a lifelong commitment set in stone, as the first members have made it to be? That their duty is to defend Equestria with their lives? That those brethren who gave their loves to protect the land have their names and actions besmirched by such pathetic little things?" His horn glowing as he stood before the door, his eyes glace back to her. "I made an oath when I joined the Guard, and I upheld it until my treachery. To serve you, defend Equestria, to fight and give my life if needed. When I open these doors, will these Guards be the same as I once was?"
"I..." She could only look down in disappointment. With no answer, the warrior alicorn gave a nod and pushed the doors open with his magic, causing a mighty swing as the hall of the barracks is reveal before the two.
"What trick is this? Where are they at?!" Confused and angry, Abaddon looks around, with not a single Guard in sight as his breaths sound heavy under his helm.
"They are enjoying treatment today." She steps up to him and gently puts a wing on his shoulders. "Please understand, we have a much better place for our Guards now. You may need it as well."
"Where are they?! Resting when they should be keeping watch?! How I would have suffered if I had done that in my time!" As he began trotting down the hallway, every door he faced he forced open. Instead of the simple furnishings of the barracks, the rooms instead resembled nurseries and playrooms fit for foals than Guards. "What are these?! These cannot be their rooms!"
"It is." She watches Abaddon in his anger and confusion. "My Guards have been so stressed and pushed even things were getting better. I turned the barracks into a safe space for them so I can ease their stress." The rooms all held furniture fit for foals, chairs and cradles, toy boxes and colorful floor mats, but they were all full-sized for bigger foals.
Finally, Abaddon reached the final room, and wasted no time bursting open the doors. What he saw in the large room before him left him silent. What he expected to be a chapel, where the Guards would go for communion and worship the Princesses they served, is in fact a large playroom, filled to the brim with toys and other things meant for foals. But what caught his eyes was who were all looking to him. Inside the room, were ponies, in fact the members of the Guard, but they didn't look like the fighting, mighty ponies one would expect to see. They were all overweight, covered with what one could say a thick layer of baby fat as they held toys they were playing with or the other things they were doing before Abaddon burst into the room. And their armor wasn't present either, with all of them in a thick, snuggly diaper taped around their waists. Some were even used or in the middle of being changed from another Guard. There wasn't a weapon in sight, at most being some play foam swords laying in a toybox. The clattering sound of Abaddons sword hitting the floor rings out amongst the silence as he puts his hoof over his chestplate, quickly turning to Celestia as he struggles to make words. "What, but... WHAT, is the meaning of this?!" He yells as he points back to the pudgy Guards in the large room.
Celestia couldn't bear to see him in such a state. "I'm sorry. This was the only solution. I created a happy environment for them and it became more than relaxation. I wanted to be somepony they could always look up to. Surely you understand that."
"This is an outrage! Where are the heroic warriors, picked from the knight Houses to serve you? Where is the honor?" Snorting in anger, he began to march up to the babified Guards sitting on the floor closest to him. But before he could do anything, he could feel himself frozen in place, unable to move as Celestia began to trot up to him.
"It seems somepony needs a treatment as well.~" Giving a smile, Celestia began to use her magic to pull off his armor.
"Release me Celestia! I do not want this!" Lashing forth eldritch tendrils in response, they were no use as his armor was stripped away, revealing a fit, somewhat androgynous alicorn underneath. He hasn't aged a day since he became one. Desperately trying to break free while some of the chubby Guards help hold him down, his eyes widened as some of the Guards were getting out things for him. A pacifier, hoof mitts, a white bonnet, and most alarming of all a thick, frilly diaper with Celestia's cutie mark on the hind end.
"I have to treat you like a foal since you are acting like one. Just relax, I know you will enjoy this." As she laid him on his back, his kicks and squirms were fruitless as he felt the puffy diaper slip under him and began to close around his waist. Surrounded by the chubby adult foals, it only took moments for him to be put into his new attire. The tapes sealed the diaper shut and his rump looked nearly twice its size, and smelled of foal powder as he was let go to sit up. The bonnet on his head made him look like a silly child, and before he could shout out in detest a pacifier was jammed into his mouth. Whatever Abaddon tried, he couldn't spit it out, and was effectively forced to suck on it as he looks around, eyes burning in anger over his situation. Celestia stood over him looking in satisfaction. "There, your new wear inside the barracks will surely help you adjust to the changes of the Guard. Please now, relax, and let yourself enjoy some stress-free fun for once."
As the other Guards gave him space, Abaddon had a miserable look on his face, pouting as he sat with his legs crossed and giving upset-sounding sucks on his pacifier. He looked like a grumpy foal, and Celestia could see it as she sat by him. "I know, you must be hungry after all of that stomping around." Levitating a bottle of milk up to his mouth, she pulled away the pacifier, strands of spit bridging between his lips and it as the nipple of the bottle pops into his mouth before he could make any sound of protest. He felt a trickle of the milk dance onto his tastebuds, it tasted sweet, hinting of vanilla and honey, the milk warm and rich. Beginning to slowly calm down as he began to suckle from the bottle, Celestia began to embrace him in her wings, gently cooing him as the grey alicorn closed his eyes and began to relax in her tender clutches. As he drinks down the milk, not only does it calm his nerves, nullifying any shred of anger he had moments before, but it also began to add some heft to him, weighing him down with a fair amount of foal pudge rendering his fit figure to a weak and defenseless look.
"There, see how good it feels to be a padded foal? No stress, no worries, only comfort and love from your mother mare.~" As she watches him finish the bottle she pulls it away and looks into his eyes while she reaches a hoof down and rubs his frilly diaper.
"How Princess, how has it come to be this?" Abaddon asks, finally calm enough to not resist her. "What happened to the Guard, that they are reduced to diapered foals than the warriors I hoped to see?"
"If you paid attention to me Abaddon, I said it was only treatment. The Royal Guard is still what you believe it to be. When on duty they are the soldiers you envision. When they enter the barracks and rest, they take on this form. Safe and in diapers I make sure they feel like the happy foals I want them to be." Abaddon looks and sees the other Guards back to their doings, playing with toys, suckling on their own bottles, all looking genuinely happy and carefree. It slowly dawned on him, how brutal and harsh the old ways were, how he remembered the endless stresses and burdens he and his brethen had to face on a daily basis. How more soothing life as a Guard would have been if they had such treatment in his time.
Laying peacefully and feeling himself unwind, Abaddon began to feel a warm flowing sensation between his legs, before realizing what it was. Celestia could see him start to panic and hushed him as the pacifier was slipped back into his mouth. A hiss was audible as he wet his diaper, the thick padding turning a light shade of yellow as he emptied his bladder out into the cushion wrapped around him. Gently stroking the back of his bonnet, the Princess slowly began to rub his wet diaper with a warm smile on her face. "Just let everything out my foal. Let all of your old stresses leave you and feel renewed as I intended you to be."
While he may have relieved his bladder, the newly adult foal could still feel something else brewing in him. Grunting as his rear made a few sounds of its own, Celestia giggled as she kept rubbing his diaper, knowing what was coming next. As he began to push, his diaper began to crinkle and sag as he began to fill it more, his face blushing red as his Princess was watching him mess himself. Looking over, he seen another Guard filling his diaper, not embarrassed at all as he was cuddling a stuffed dragon. Another was busy changing one, both happily giggling like a pair of schoolfillies throughout the diaper change. Having emptied himself into his now full, and smelly diaper, Celestia smiles as she puts her hoof under his rump, feeling his mush tush sagging from under him and gently removing his pacifier. "How do you feel now Abaddon?"
"Warm and comfortable. Safe, mother..." In a state of bliss, Abaddon felt happy and safe as he sat in his used diaper. His wings spread out, basking in the warmth and care Celestia provided him.
"Welcome back home Abaddon, I knew you would accept the chance relax yourself. Now, how about I change you and get you nestled in your new room for a nap?" Helping him onto his hooves, Abaddon seeing now a bit more difficulty moving with his added weight, she began to lead her foal to his room to begin his duties once more in his renewed service.