[Yaoi, Diaper Use, Soiling, Diapers, ABDL]

The most precious time of year was about to roll around. At least, to the Servants of Chaldea, that is. Their beloved Master, the young man who had summoned them to help with the Grand Order incident, was about to grow in age. Indeed, his birthday was just around the corner, and most of the Servants were in a rush to figure out what they should do to make him feel just how much they loved and needed him.

One of these Servants was the Lancer-class Karna, who had trouble thinking of something that would properly fit a human such as him. For that reason, he had turned to the Caster-class Leonardo Da Vinci, who had the reputation of knowing exactly what a boy like him might need.

He just didn't expect to be shown a video of the young black-haired man doing... naughty things in his spare time. Not to mention, wearing nothing but... something rather puffy around his waist.

"What... exactly am I looking at, Lady Leonardo?" The formal Karna asked as he turned his head towards the brunette deviant, who merely giggled in return.

Instead of giving him a straight answer, the Caster handed him a set of different illustrated books, smirking all the while. "I'm just showing you Gudao's interests, Karna. He likes playing around in diapers, buuuut... All the footage I've captured of him having fun seems to be lacking a little... something. I think filling the role of that something will be the best gift you could possibly give him."

The white-haired Lancer looked down at the books, noticing the excessive amount of male-on-male romance displayed within, coupled with those strange 'diapers' that the devious Leonardo spoke about. He did also notice the presence of a paternal figure within every single book, slowly making the gears turn within his head. "...May I borrow these for a little while, Lady Leonardo? I have to study them."

Da Vinci smiled and waved the white-haired Servant off, prompting the latter to leave with his mind made up. He had the perfect gift in mind now, and his Master was no doubt going to enjoy it. Every single second of that experience...

---

Days later, on the night before the Master's birthday, things were getting quiet. Each Servant had prepared their gifts, so it was just a matter of waiting until daybreak came for each of them to ambush him and give him all of their love.

With that in mind, the young man had a plan to spend the entire night all to himself. He knew that he wouldn't be able to enjoy himself like this again any time soon, what with all the gifts that he had to enjoy, and the duties that they had to tend to. So, like the little baby that he was at heart, he was going to make the most of the time that he had.

Little Gudao laid in his bed, squeezing his thighs together as he focused on the softness covering his butt. He giggled a little into his pillow as he pushed his hand into the side, watching as his fingers practically were swallowed by the poofy underwear. He had picked the poofiest and most squishy diaper for this occasion, hoping to use it as much as possible. He had even prepared a special spell, one that Da Vinci had taught him so that cleanup wouldn't be difficult. He'd just have to cast the spell, and everything would disappear. Nobody would realize that he liked wearing diapers...

Or worse than that, that he liked to use them as much as possible. The young Master closed his eyes as he grunted a little, pushing and pushing as he felt the back of his padding grow heavier. He could feel the laxatives working, as he managed to fill the back with quite the impressive mess. He could feel the warmth and the dirtiness coating the inside of his thighs, but he didn't care. Tonight was his night.

The little baby boy chuckled as he sat up in bed, feeling the mess underneath him squish around in response to his movement. It felt so nice to have such a dirty mass rub up against him, to the point where the front of his poofy diaper started to distend just a little. He could feel his rock-hard erection through the padding, but he didn't want to touch it just yet. He wanted to edge as much as possible before he'd try to cum, that way he could savor the experience that much more.

Or at least, that's what he would've done if not for the sound of knocking reaching his ears. He felt a sense of dread overwhelm his figure as he slowly turned his head towards the door to his room. Who could be visiting this late at night? Who could... Who would try to ruin his fun? Did he really have to use that emergency spell right now? How was he going to explain the boner that was trying to push through the fabric... or the stench that was filling the room for that matter?

"What a naughty little baby..." A whisper graced the young man's ear as he froze in place. He wanted to look behind him and figure out who was talking down to him as if he was an infant, but the sensation of being caught had robbed him of any autonomy. He was just forced to sit there, in his own mess, as somebody started to humiliate him...

The gentle hands of somebody older than him slowly moved their way onto his diaper, pushing against the mess that had gathered around his bottom. "My my. You really made a mess, didn't you? Daddy's impressed with his naughty little baby boy..." The voice was smooth, almost supportive even. There was only one man in the entirety of Chaldea who could have that kind of voice...

As Gudao realized who was babying him, his erection grew harder. He could feel his body tiptoeing over the edge, but he couldn't cum. As if his body visibly refused to cum unless his Daddy permitted him. This was what he had been waiting for, after all. He had been wanting a Daddy for so long...

The erection wasn't lost on his Daddy, who continued to whisper into his ear. "Does Daddy's little baby want to cum?" Instantly, the black-haired boy started to nod. "Good. Good baby boys get what they want." The young man could practically imagine his Daddy's smile, as he suddenly felt a hand slide across the bulge in the front of his poopy pampers...

Instantly, he felt all of his resistance and his worries melt away into a slurry as he thrust his hips into his Daddy's hand. He could feel the orgasm wracking his body as he let out a loud moan, his tongue hanging out of his mouth as the front of his diaper quickly grew soggy from the cum that it soaked up. It was actually a little impressive that he managed to cum that much, especially considering the size of the padding wrapped around his waist.

And he wasn't done either, as he felt his Daddy's supportive hand push his mess again, making another jolt of pleasure shoot through his entire body. It didn't take long before he thrust his hips forward again, entirely out of instinct, as he released yet another splurt of cum straight into his padding. He was far too turned on by all of this.

"You're Daddy's favorite little baby boy, Master..." His Daddy muttered, with that last line breaking the illusion. The young Master managed to turn his head to face the white-haired Karna, who immediately started to blush now that the ruse was broken...

The Lancer-class Servant was at a loss for words as his eyes fell upon the absolutely adorable blush that coated his Master's face, and the way that he pouted over being used like that. "Master... Forgive me for acting that way. Lady Leonardo told me about your interest, and after studying the kind of pornographic material that was all about this kind of relationship, I thought that you might enjoy being spoiled by a fatherly figure. I... would not consider myself good at the role, but I did what I could to make you feel satisfied..."

As his Servant explained himself, Gudao continued to look straight up at him with his lips still forming a pout. He closed his eyes, only to push as hard as he could down below. He pushed and pushed, making the back of his diaper bulge out a little further as he packed it with another few ropes worth of filth. Only after the back of his padding sagged its way onto the surface of his bed, did he finally speak up.

"I-If you're so sorry about being my Daddy, Karna, then you're going to have to keep up the act. You... You did really well, and..." The young Master muttered, his embarrassment still pouring through every word he uttered, He couldn't scold his Servant like this, especially since he did enjoy all of it. "...and I need a change before morning rolls around..." He concluded, before just hugging his newfound Daddy.

Karna smiled as he hugged his little baby boy back. The events of that night would remain a secret to the two of them, as their relationship deepened. Not as Master and Servant, but as Daddy and Baby.