Notes

A new class is instituted for the 4th Holy Grail War - the Slut, one possessing absolutely no power whose sole purpose in the war is to be used and fucked. Their Noble Phantasm is unique in that it is the same for every Slut: the ability to come infinitely. That is not to say Sluts do not get fatigued from sex, but rather that, despite this, they are still able to come.

Waver realized, as soon as the smoke and mist faded away, his Servant was a Slut. Undoubtedly the weakest and most underestimated class, Waver was nonetheless overjoyed - because everyone viewed Sluts as naught but toys to play with, it afforded Waver the chance to get rid of the Master while his or her servant was busy fucking the Slut.
His servant was massive, towering over Waver with gleaming muscles and a perfectly sculpted body which Waver could see because, as Sluts traditionally were, he was summoned wearing absolutely no clothes. Waver grinned slightly - the seventh Servant, Caster, hadn't been summoned yet, which meant it was unlikely anybody would move against him. He could take a few days to break in the Slut.
When the Servant realized, belatedly, he was wearing no clothes, he blushed slightly and moved to cover his crouch. His red hair and beard, Waver reflected, matched well with the expression.
"T-t-tell me, are you m-my Master?" The Slut managed to grit out as it shivered in the cold of the night. His bulging arms wrapped to cover his chest and rough hands clutched his rather large shaft.
Waver stepped forward confidently - not even a normal human or mage had reason to fear the Slut, for he was so completely devoid of strength he likely couldn't beat a 5-year old in a fight. "My name's Waver. And who are you?"
He smirked as the Slut's blush deepened. "Y-you bastard. You know who I am."
Waver shook his head and took another step forward. The Slut inched backward minutely. "No," he replied, "I don't. Or do you want me to keep calling you 'Slut'?"
The Slut sighed and angled his head to stare at the floor, obviously ashamed of his name now that he was nothing but the Slut. "I was formerly Iskandar, King of Conquerors."
Waver nodded his head. "Good, good, and tell me, Iskandar, what is your class?"
Iskandar's blush deepened (if that was even possible) and he looked away as he muttered something under his breath. Waver grabbed his chin between thumb and index finger and forced him to look at him as he repeated the question firmly.
This time, though Iskandar's voice raised only slightly, Waver could hear him very clearly say, "I am the Slut."
"Then you know what you should be doing right now." Waver stepped back and gave the hulking man a raised eyebrow. But the Slut only shook his head vehemently and continued to back away.
"No, no," he shook his head as his naked feet pressed against damp grass, "even if the Holy Grail chose me for this role, I am under no obligation-"
"You're a servant," Waver interrupted loudly, "so you know as well as I do you are." Iskandar stopped reluctantly and clenched his fists in frustration. Waver approached him once again, this time hastening to grab his firmly muscled arm before he tried to escape again. "And you also know the Slut's Master has an unlimited amount of command seals to compel obedience no matter what the Slut actually wants. Running or resisting is pointless." He pointed at his groin. "I was going to go easy on you and just have you suck my dick, but you've enraged me. I want you to touch yourself."
Iskandar choked and looked around. "Here?" His deeply masculine voice betrayed his disbelief. "Right now?"
"You're going to be fucked in places much more public and worse than here, so you may as well get used to it now," Waver said dismissively.
His free hand clenched even tighter as his other wrapped around his half-hard cock and pumped once, reluctantly. A moan spilled from his mouth before he could help it.
Waver raised an eyebrow. "Oh, sensitive, are we?" He tweaked a nipple and Iskandar roared reluctantly, his whole body quivering from the slight motion. The naked man stood there, shivering from both the cold and shame, but was brought out of his trance by a slap to his muscled chest. "I didn't say you could stop, did I? Do I have to use a Command Seal to make you touch yourself?"
Iskandar growled but complied, continuing to pump even as each movement drew a whimper or groan from him. He hated every moment, but he knew being forced against his will to do it - watching his body fondle itself without his permission - could only be even more humiliating. As he slowly but surely teased his cock, Waver expertly played with his nipples, licking and twisting them to hardness. He tried to conceal his moans with a burning face, but Waver only hastened his efforts when he noticed Iskandar's silence. His inexplicably talented fingers danced across the older man's chest with ease, touching precisely each of his most sensitive spots with a light tap that left Iskandar sobbing with pleasure. Literally. Tears ran down his face in streams and rivulets, his body trembling with sorrow even as his hands continued to tease at his cock. They dripped down onto the grass from his chin and joined the pool of pre-come made by his cock.
He could feel it, feel the disgusting sensation of slick pre-come against his wrist and palm as his cock continued to spurt helplessly onto his hand. The white liquid ran down his red length, and then proceeded to his muscled legs, continuing down in the various dips and curves of his developed muscles until finally spilling into a small area with little grass.
It built up, like a pressure in his cock, slowly but surely. His sobs intensified as his cock grew harder still, and Waver's hands continued to mercilessly tease his chest and nipples. His Master had just made his way to his perfectly defined abs when he finally came with a half-roar, half-sob that left him lurching over into the grass, writhing around as his cock spurted seemingly endlessly, covering his chest, abs, and even face in long, thick ribbons of come. His arms spasmed as his back arched, his entire body clammy with the effort of orgasm. Finally, Waver's hands left his chest, and he could've sobbed in relief had his vocal chords not been preoccupied with forcefully belying his shameful pleasure. When the orgasm finally stopped, Iskandar collapsed in the bed of grass, feeling the come slowly sliding off his flawless skin and dripping into the grass. His cock remained hard, but he felt drained and exhausted.
Suddenly, there was a hand at his cock, stroking despite his sensitivity and exhaustion, and he began to writhe even more erratically as more come began to spurt from his cock, which wripped around in his flurry of movement. The hand continued to milk the come from his cock for 10 more seconds until Iskandar was crying, literally sobbing with pleasure, his moans intermingled with hiccups and gasps for mercy.
Another hand slapped his ass, and he shifted slightly despite his exhaustion as the small hand fondled without even the smallest hint of shame his toned and muscled ass.
"Oy, Rider, you got some on my clothes. Lick it off."
Suddenly, there was a sweater in his come-covered face.
"And you're filthy too, we'll have to give you a bath." Despite Waver's tone of disapproval, Iskandar, faint and light-headed as he felt, could sense the smirk hidden underneath.
And after he'd licked the come off Waver's clothes (he'd had to use 3 command seals to get him to complete the job, and, of course, by the end there was a fresh layer of come on Iskandar's body because Waver couldn't just let him go unpunished for dirtying his clothes in the first place), he trekked back to the house Waver was staying at, where he was subjected to a pleasurably tortorous bath that involved many a wandering hand and eyes, a muscled, writhing body splashing water all over the place, and one exhausted Servant, quivering under his cruel Master's touch and whimpering for help as Waver whispered to him to come for a 11th time. He could feel the pool of come literally floating around him, coating his legs, cock, and abs in a substance composed more of his come than actual water. He felt dirty, filthy, ruined, and utterly ashamed.
Waver grinned and continued his ministrations. He intended to force Iskandar to come at least 100 times by the end of the night. He, after all, had to break him in. As the massive man underneath him roared once again, his voice hoarse from overuse, and spurted more come onto the wall behind him, his arms thrashing around in his own come, his hair soaked with come that dripped back into the pool, he wondered which Servant and Master would come first. Hopefully, Waver thought as his hands scooped up a pile of thick come and smeared it onto Iskandar's chest, they would be willing to share.
Waver left Iskandar in a bathroom utterly covered in come to change his clothes. Of course, he intended on coming back to finish the job - he just needed a quick break first. The large, muscled man sunk in the pool of his own come, utterly incapable of moving even an inch, every single inch of his body soaked with come, his ass, red and sore with those spanks Waver had given him when Iskandar had tried to escape. He was not yet resigned to his fate as Slut and cumdumpster.
But he would be soon.