Speed O' Sound Sonic had been kicking himself for his failure against the heroes. He'd let himself be caught of guard, which had never happened before in his entire career as an assassin. If Spring Mustachio had been even a bit less keen on taking him in alive like a good little hero, Sonic would be dead right this very moment, and that was more than Sonic was willing to accept without fighting back.
He had to lay low for awhile, healing and training until he was not only back to top condition, but was even better, and more importantly faster, than he had been when he had been hit by that A class's special attack. With his skills honed, and his weaponry sharpened, Sonic set to planning when and where he would take his revenge. Obviously he would be targeting the association HQ, but when would both of his targets be there..
Of course, evaluation day! Iaian and Spring Mustachio would both be present to show the association their skills in an attempt to earn a higher ranking. Sonic would be there waiting for them, and neither of them would leave the facility alive. As an extra bit of bile against the association, he supposed it couldn't hurt to break a few random villains and monsters free as well. Yes, it was going to be glorious.
Sonic made his way to city A, waiting for the right moment. He got there just as Spring Mustachio and Iaian were entering the building. He rushed in passed them before the door shut behind them, too fast for either of them to notice. He could have killed them right then and there, but doing so would have cut his time down critically and raised more alarm than he was comfortable with until he was on his way out.
He ran through the facility, focusing on sustaining his speed so he would not be picked up on camera, and on scouting out the area in search of the holding cells for the criminals and the few monsters they'd decided to capture alive for study. He found them quickly, then made his way to the judging room. It would be a quick, clean kill of both of them, then down to the cell room to cut all the locks, and then out of the building before anyone knew what was happening. He'd be a mile before the heroes' heads even hit the ground.
Or that had been the plan anyway. He knew that he was going to have great difficulty carrying any of it out the moment a foot caught his leg and sent him flying into a nearby wall. At the speed his was moving, there was no way he'd just accidentally lost track of where his feet where. If he had, the person's foot would have been cut off and he wouldn't have even noticed until he'd felt their blood on his ankle. No, this had been deliberate. Someone had seen and tripped him up.
Turning around, Sonic saw a man-or he assumed it was male, though the alternative wouldn't have been shocking-dressed in a white cloak, with platinum hair and an emotionless expression on his face. "Who are you, and what are you doing here?" He questioned, his voice confirming his gender. Sonic grit his teeth, "None of your concern hero. I'm here on business, just walk away, and there won't be any trouble."
Normally he wouldn't be able to let go of someone being able to see him when he was moving that fast, but he held greater anger for the heroes who'd managed to stick him like a pig, and didn't want to waste any time if he didn't have to. The hero's expression didn't change as he said, "I'm afraid that isn't how this works. You're clearly not meant to be here, and if you were moving that fast, it means you've got powers. I don't expect a hero was sneaking into the association, so," He moved his cape aside, revealing a sheathed sword at his side, "What are we going to do now?"
"We aren't going to do anything," Sonic said, vanishing from view as he ran at top speed, appearing behind the hero with his sword already drawn, "You're going to die!" He brought down the sword in a powerful slash that would have been lethal, had the hero's sword not risen to block it. Sonic bolted back to the far wall, a look of shock on his eyes. He hadn't even seen the man move, and now his sword was fully drawn. It was a single movement as well, eliminating the possibility that it was just a special ability like Iaian had used.
"Who.. who are you?" Sonic questioned angrily, gripping his sword tighter. "S class rank thirteen, Flashy Flash." He said calmly. Sonic had heard of King by reputation, and anyone who'd ever been in prison in the last three years knew who Puri-Puri Prisoner was, but Sonic didn't know the actual names or faces of any of the S classes, save perhaps for Tornado, who he'd once seen push back a storm with nothing but her mind. He was perfectly fine never going up against anyone in the top ten of the S class heroes, but he refused to accept that he had been outpaced by rank thirteen.
'Luck,' he told himself irrationally, 'pure luck, nothing else.' Sonic sheathed his sword, reaching into his pockets and letting out a storm of smoke shurikens. Flashy avoided them all, but the smoke they let out filled the hall. Sonic took out his sword again, charging forward with his Full Frontal Attack. He knew no one could keep up with him at this speed. The only attack faster was his Ten Shadow Burial.
He was proven wrong when at no point did his sword make contact with anything, forcing him to grind his heels into the floor to avoid crashing right back into the wall. As the smoke cleared, he saw that Flashy-Flash was now on the other end of the hallway, unharmed and unfazed. He practically growled in frustration, reaching for his exploding shurikens to use Hail of Carnage, but not finding them.
"Looking for these?" Flashy asked, holding out his hands, showing that he was holding all of the exploding shurikens, "I took them off of why as you ran through the smoke. I wondered if you'd notice." He said, showing he'd also taken the regular shurikens and the kunais. Sonic had nothing but his sword and his own two feet. He gulped now, creeping dread beginning to fill him as he realized that this was different than before. He wasn't fighting an A class with a temporary ability, this was an S class, with real speed.
"How are you-" Sonic's words were cut off by the Flashy Fist, one of Flashy Flash's attacks. In what seemed to Sonic like an instant, Flashy went from one end of the hall to the other, launching into a barrage of punches that hit Sonic's face and body too quickly for him to counter or dodge. The combo attack ended with a powerful backhand that sent him to the floor.
Flashy drew his sword again, holding it aloft in warning, "Stay down, it'll be better for your health." "Quiet!" He hissed, again vanishing and appearing at the wall above Flashy, jumping down and attempting to strike his shoulder with his Wind Blade Kick. There was a momentary look of surprise on Flashy's face, which Sonic has assumed meant Sonic had outpaced him; until Flashy responded by grabbing his leg and throwing him across the hall.
Flashy zoomed at Sonic with his famous Flashy Slash, which was normally his go-to finishing move, a lethally quick slash to the head that had never been dodged. And it still wasn't dodged, but it didn't kill Sonic. Flashy pulled the strike at the last moment, the tip of the blade just barely breaking the skin of the assassin's forehead. Instead, Flashy struck Sonic with own variant of the Wind Blade Kick, knocking him unconscious instantly. It had been seeing that someone else had mastered the move that had caused his surprise.
Sonic came to in chains. His arms were held behind his back by a mechanical lock, his legs bound to the floor, and his body to a chair. He'd been taken to a featureless grey room, likely one more high security, as they'd no doubt recognized the threat level of the villain once Flashy Flash brought him in for identification. Just before Sonic would have launched into a barrage of frustrated curses at no one, or perhaps everyone, in particular, he realized he wasn't alone in the room.
Flashy Flash was standing against the wall, likely having been waiting for Sonic to wake up. "You're fast." He said plainly. "Evidently not fast enough." Sonic said bitterly, "So much for being the fastest human in the world." "Is that what you thought you were?" Flashy couldn't help but grin in amusement, only adding to Sonic's dislike of this entire situation.
"Is that why you're here? To mock me and congratulate yourself on your victory?" Sonic questioned. Flashy Flash shook his head, "No, I'm here because, easily as you went down, that was still the closest anyone's gotten to landing a strike on me in quite a long time. I'm impressed." "Please, impressed with what? That was a mess." Sonic said, unable to see anything positive in a failed attempt.
"By your speed mostly, though your tactics and combat skills aren't to be overlooked either," Flashy Flash said, "If you took the heroes exams, you'd likely make A class instantly, and be S class in just a few weeks, if that long." "Maybe so, what of it?" Sonic questioned, "Are you actually here to try and talk me into being a hero? Do you honestly believe I care about who lives or dies in this world?"
"Based on what I read in your file, I'm quite certain you don't," Flashy Flash said, "But I'm also certain from how you act and speak that you do care about your speed, quite passionately in fact. And if you were to join the association, well, I could train you to become as fast, if not faster than myself. Or you could stay here and get shipped off to prison. I'm sure Puri-Puri will love you."
Sonic glared at Flashy Flash, both already knowing what the eventual outcome would be.
So yes this strays a bit from the basic concept of 'monsters/villains Saitama fought fighting other heroes', but I got the idea and thought it was interesting.