They've been at this for hours, and Leonardo hasn't flagged. His hands are shaking slightly, his eyes are narrowed with concentration, he's started slyly poking bruises to keep himself from falling asleep...but he hasn't asked for quarter, hasn't even asked what the purpose of this exercise is.
Saki has set up a system where he alternates between fighting his chunin himself or sending in groups of Foot soldiers while their master rests or even naps. He's long since exchanged his armor for lighter black clothing, for better maneuverability and endurance.
Leonardo has lost every fight against him, and won against everyone else. He's kept up the same steady pace, never wasting a movement or missing an opportunity, long after most of Saki's other soldiers would have begged for mercy and simply collapsed.
Shredder sent Alopex and Karai into the ring at one point, and watched them both be bested. Karai had stalked out of the dojo in a huff, while Alopex had just looked relieved that she would be allowed to get some rest tonight.
Still, they were the only ones other than Shredder who've managed to wound him so far. There's a particular nasty scratch on his arm that's been allowed to go untreated, and Leonardo has long since stopped wincing every time he moves that arm. Many of his opponents have lunged at that vulnerable spot, but he's managed to turn it into bait for a fairly effective trap.
It's not that he doesn't feel pain or exhaustion, or fear or guilt, Kitsune told him, explaining what she'd done to Hamato Yoshi's favorite brat. It's just that none of that matters nearly so much as pleasing you and protecting the clan. I really didn't have to do that much core tampering, to be honest. Just switch his loyalties, turn you into his savior instead of a threat. 
And oh, she has done a magnificent job. The child who used to be so irritating has now become one of his greatest soldiers; more effective than Karai, more ruthless than Alopex, perhaps even more loyal than Kitsune. To see him twist and strike with glittering blades and nimble fists, clad so prettily in Foot colors...it almost brings a smile to Saki's face. Almost.
Leonardo ducks nimbly under an odachi, sword twirling to knock the hilt out of the wielder's hand, and the memory of Yoshi performing the exact same move strikes Saki to the core. For all that he should be a lumbering hulk, Saki thinks, his chunin still manages to show a surprising amount of lightness and grace, the kind that reminds Saki of Yoshi even though they look absolutely nothing like each other now.
It's...intriguing, to say the least.
The trust in Leonardo's face reminds him of Yoshi, too, from back in the early days before Yoshi had started questioning everything he did, back when they were still sharing gentle kisses in the dojo and hot-blooded trysts in the woods, back before the discovery that had shaken Saki's life apart, back before fire and blood and his own sword in his guts.
He wonders, idly, how far he could push that trust.
The latest batch of poor souls staggers out of the dojo, groaning, and Leonardo's already shifting into a combat stance. Is he swaying a little? Maybe. It's hard to tell. His dark eyes glitter in the low light, bright and ready.
He's gotten better since that day on the rooftop--good enough to stay out of Saki's grip for minutes at a time, shifting and lunging with a razor-sharp focus. He lands one blow, then another, before Saki finally sends him flying into a wall. Leonardo immediately jumps back up and starts attacking again, because Master had not given the order to stop.
His chunin falls two more times before Saki finally brings the fight to an end. He summons the night's final batch of ninja, although Leonardo doesn't know this. To him, it's just another group of strangers in black.
Leonardo turns to face his new opponents, but Saki grabs for his mask tails, gently tugging him to a stop. He imagines that one or two of the ninja might be raising an eyebrow at the sight from behind their masks, not that he cares about that.
"Sorera o korosu," he murmurs in his chunin's ear, and the little turtle stiffens ever so slightly.
Kill them. 
Leonardo does not protest, does not question. But Saki thinks his hands might shake just a touch more as he draws his swords one more time and rounds on the men.
They don't notice, of course. They're too distracted by his green skin and impossible shape, too busy thinking of ways they might beat and humiliate the freak. That is one of Leonardo's strengths--the strength of all mutants, really--the ability to keep people so focused on what he looks like no one gives proper thought to what he might do. Not until it's too late.
When the first one's head pops off in a crimson spray, they all freeze for a split second, and that's when Leonardo runs another through. Their eyes fly to Shredder for a heartbeat, wide and panicked: no one dies in training, at least not big, important members of the Foot Clan like them. Don't they have Karai's protection? Doesn't that mean anything?
They see something cold and vicious in Saki's eyes, and that's when they realize Karai's protection means less than nothing.
The men put up a good fight, they really do. One even lands a little scratch on Leonardo's shoulder, right before his arms are chopped off and his throat slashed open. Another actually manages to knock a sword out of Leonardo's hand and receives a shuriken through the eye and into the brain for his efforts.
None of them bother to scream. No one will come near them even if they're near enough Shredder's private dojo to hear, which is doubtful. They use all their breath for fighting and it's still not enough.
Leonardo paints the blood red with their guts and Saki finds it beautiful. Saki finds Leonardo beautiful right now; the sight of his little pet monster cutting down men like so many trees makes his breath catch.
(It might also make him hard, just a little. A side effect of wandering around an endless battlefield in the underworld, when the only things for him to fuck were insane dead soldiers)
At last his chunin stands, panting, in a ring of corpses. Saki's lips twitch with amusement as Leonardo jerks his head in an almost-bow, before remembering there's no one left to exchange such courtesies. He plucks a rag from his belt (left there for just such a purpose, although Saki doubts Leonardo realized that until now) and carefully cleans his weapons before sliding them back into their respective sheaths.
Then he throws up.
It's quite impressive; a projectile spray of rice and pizza chunks arcing across the room, landing in the gore with delightful plopping noises. Saki raises an eyebrow at the sight. "Having trouble, chunin?" 
"Master!" Leonardo whirls to face him, and Saki's surprised to see fear in his eyes for the first time this night. "I wasn't--I didn't mean--oh!" His quivering foot slips in a pool of blood and he's falling, only for Saki to dart across the room and snatch him up.
He could simply pull Leonardo to his feet, but he prefers to tug him up into a bridal carry instead, to feel his chunin's pulse thrum-thrum-thrumming against his chest. And look at that, he's blushing. Saki hadn't even known the little beasts could blush.
"I--I'm sorry," Leonardo grits out, muscles locking with the urge to scramble out of Saki's arms immediately, but he can't just do that to his master, can he? "I, that was disgraceful, I apologize, I..."
He's terrified of being proven unworthy, of being sent back to the rat, Kitsune had said. The shock's probably not helping matters either. His eyes are so huge right now, and the sight makes him look more like a young Yoshi than ever.
"Hush," Saki soothes, placing a soft finger to Leonardo's lips before he can babble anymore. "It's all right. I'm not mad."
Leonardo blinks. "You're not?"
"The first time is always difficult," Saki says, setting him back on his feet. "I'm proud of you for handling it as well as you did." Pride is something he and Yoshi dole out carefully, the way their masters did for them.
The light in Leonardo's eyes is far too visible--gratifying enough at the moment, but he'll have to train it out of him later. A good leader knows to always hide his weaknesses.
For now, though....he's still blushing as he shifts into a little bow, careful not to clunk heads when Saki kneels before him. "H-hai, Master."
Saki places a hand on his shoulder, feeling how the muscles quiver under his touch, but do not pull away. Leonardo stands in a room full of men he's just butchered and the only thing he has eyes for is his master; further proof of how completely Saki owns him. And suddenly, he wants everyone to know it.
He dips his hand into a pool of blood, making sure not to touch anything with vomit mixed in, and tugs Leonardo's head up so that their eyes lock. His grip tightens with bruising force as he drags his fingers across his chunin's mouth, smearing them brighter than the finest rouge. Leonardo doesn't flinch, even as his eyes narrow slightly with confusion.
When Saki shoves his tongue down his property's throat, tasting spit and the blood of strangers and gloriously acidic after-vomit, Leonardo doesn't twist away. He doesn't exactly reciprocate the kiss, either, but Saki puts that down to the shock.
Saki pulls back and smiles at the sight of Leonardo's chest heaving, that pretty blush worse than ever before. Yoshi reacted much the same way after their first kiss--and then he scampered off, giggling like a little girl. Saki shoves the memory away angrily; he has neither the need nor the patience to play such games.
"Chunin," he growls, holding up a hand. "I seem to have gotten my fingers dirty."
"Hai," Leonardo says, reaching for a rag in his belt, but Saki shakes his head. "You will not need your hands," he says, pressing his fingers up against Leonardo's mouth.
He blinks, first in confusion and then in comprehension, before opening his mouth. He tries to start at the very tips of the fingers, but Saki shoves them a little bit deeper with a significant look, careful not to trigger the boy's gag reflex.
Leonardo laps and sucks rather clumsily, but with a will that sends a fresh flash of heat straight to Saki's groin. He'd known that his chunin was a total innocent beforehand--known that Hamato Yoshi had been too foolish to take advantage of his son's inherent need to please--but it's quite another thing to see it in action.
When his hands feel clean enough he tugs them free, leaving the turtle swaying, his lips smeared with blood and spit. God, the idea of those lips wrapped around his cock...but no, not tonight. Later, perhaps, after Leonardo is more properly broken in.
Quick as a striking snake, his hand wraps around Leonardo's mask tails and yanks, sending him crashing to the ground so that he lands shell-first in a pool of blood. He immediately starts to scramble back up with what Saki thinks is a look of relief--this, at least, is something he knows.
Only it isn't, because Saki places a gentle hand on his chest, stopping him while he's still sitting up. "No more sparring," he says, carefully pushing Leonardo back onto his shell. "This is a different kind of training." Leonardo opens his mouth only to close it with a snap; he knows there's no arguing with Master.
Saki smiles at the sight. "Good boy." He's still got a hand full of Leonardo's mask and carefully tugs, sliding it up off his forehead before setting it off to the side, away from the mess. He does the same with the rest of his chunin's gear, taking his time, running his fingers over green skin that's so smooth and callused in equal measure, poking and prodding at carefully honed muscles. He lets his touch linger without shame, because this turtle is his, and a man should be allowed to touch his things however he likes.
It's only after everything is put away that Saki notices how Leonardo has gone stiffer than a board, gaze locked on the ceiling, tense and quivering. That won't do. "Look at me, chunin, " he orders, seizing Leonardo's chin and turning his face until their eyes meet. Leonardo's jaw clenches, but he doesn't look away.
"Relax," Saki orders, carefully massaging Leonardo's arms and legs. "Relax." Leonardo does as he's told, or makes an effort to, perhaps hoping that letting his master strip him like this will be enough.
Only possessions do not have the right to say "enough," or even think about it. Saki knows that. The question is whether Leonardo can learn it.
He sits back on his knees and gives one more order: "Spread your legs."
At this, Leonardo freezes; innocent as he is, there are some things which need no explanation. "Master?"
"Do I need to repeat myself, chunin?" He lets a bit of disapproval creep into his voice, and Leonardo crumbles almost immediately. "Hai, master," he replies, opening his legs so that Saki can shift to kneel before them.
"Wider," he says, and again, "Wider," until Leonardo's heels are digging into the dojo floor, his muscles trembling with effort. The sight is so much more satisfying than if Saki had yanked them apart by force.
He dips a hand between his chunin's thighs, smirking as the boy's breath hitches. The skin there is soft, very soft, with a little slit that jumps at his touch and beneath that...."Do you have a tail, Leonardo?"
" Hai , Master." There's a bit of a plea in Leonardo's eyes, but Saki is having none of that.
"Let me see it." He's disappointed that he has to ask, and he lets it show. Leo winces, but carefully unrolls his tail; it's longer than Saki expected, and a lighter shade than the rest of his body.
He rolls it through his fingers, marveling at how it flutters at his touch, and is rewarding with a soft....purring? sound. Interesting.
Leonardo blushes worse than ever. "Sorry, master--"
"Do it again," Saki commands. He squeezes even harder and Leonardo's legs flex, his fingers scraping across the ground. The sound he makes thrums up through Saki's hands, tingling pleasantly, and there's a small bulge in his plastron.
Still keeping a careful grip on the tail, Saki bends to examine Leonardo's pink, clenching hole. Leonardo squirms at the sensation of eyes on such an intimate place, and Saki puts another warning hand on his plastron, making sure that the edge of his palm just brushes Leonardo's slit. The little turtle is practically vibrating now, no matter how hard he tries to stay still.
They are surrounded by corpses; Saki doesn't have to reach around that much before his fingers are slathered with thick red globs. Leo bites his lip as Saki's fingers drop back between his legs, trail up along his tail, to the base....
When the first slick finger slips inside him Leonardo jolts, almost as if on instinct, before forcing himself back down with a grunt. He digs his fingers into the dojo mat, no doubt battling all the instincts that scream at him to resist such an invasion.
"Very good self-control," Saki says, wiggling his finger around. When there's no response he thrusts a little deeper, scraping against tight, hot inner walls. "What do we say to such compliments, chunin ?"
"Ah! T-thank you, Master."
While one hand works away, the other is still stroking and patting at Leonardo's tail and slit. Saki can't keep from smiling at every little jerk or twitch Leonardo makes, every animalistic sound. At heart, he's just a good little beast making pretty noises for its master.
"Come out," he croons, noticing the way Leonardo's slit is clenching and the look of concentration on his face. "You know how you want to." Leonardo obediently spills into his hand--he's got a rather freakish cock, but the fact that Saki can see it at all is the real victory.
He flicks his tongue along the tip and is rewarded with a yelp, making sure to squeeze the base so the brat doesn't come early. He and Yoshi had done that far too many times in their younger days.
Yoshi, Yoshi, Yoshi.....What would he think if he saw his precious little leader now, moaning like a whore in the arms of their greatest enemy, the man who had literally killed him? Would he be disgusted? Self-righteous? Would he blame Saki for his lust and Leonardo for his weakness?
Or would he want to join in? Saki likes to (has to ) think it would be the second one.
He's scissoring Leonardo open now, his movements quick and brisk, impatient. Everyone else is dead, but they are alive, and that life is boiling in his veins and hissing in his brain, desperate to go somewhere.
Leonardo whimpers as Saki adds more blood and sinks deeper, deeper, until he finds himself prodding something that triggers a soft gasp. "Feels good, doesn't it?" Saki asks. His only reply is an incoherent groan, and for some reason there are tears in Leonardo's eyes; Saki growls at the sight, reaching up to roughly wipe them away. "It feels good, chunin , does it not?" he hisses.
"Ye-yes, Master." Leonardo's voice sounds odd, almost strangled, ringing with a mix of pain and pleasure. Saki jabs again and is rewarded with a shrieked "Yes!" He does his best to ignore how much it sounds like a wail.
Leonardo likes this. Leonardo should be--Leonardo is-- grateful for his master's attentions. And if he isn't Saki will make him grateful.
He tugs his cock out of his pants--Leonardo must earn the right to see him fully naked, the way Kitsune did--and brushes it down Leonardo's plastron. His chunin's eyes go huge, panicked by the size. "Master, I can't--"
"I have faith in you, chunin." And without another word, he lines up and shoves himself in.
There's a bit more friction than he would have liked--perhaps blood was not the best idea, perhaps he could have spent more time preparing--but it's still so good Saki doesn't care . Leonardo is so tight and snug and it feels like he's being sucked in hungrily, like Leonardo is as desperate as Saki suddenly feels. The little turtle lets out a soft groan, his cock still weeping precum in Saki's grip. 
Leonardo's limbs scrabble along the floor, like a trapped bug, and Saki doesn't like that. "Hold on to me," he orders, and Leonardo does, clinging to him for dear life. When Saki hits that little bundle of nerves, another purr vibrates through both their bodies.
Saki manages to hold still when he bottoms out, letting Leonardo adjust. He pulls him close, until their faces are inches apart. "You do not deny me," he whispers, voice soft as a lover's and unrelenting as a god's. "You do not even think of denying me, ever." He gives Leonardo's cock a cruel twist, driving the point home. Leonardo's jaw clenches but he manages not to cry out.
" Hai, master," is all he says. All he will ever say, that truly matters.
Saki begins to move then, to lose himself in heat and scent and soft, strong flesh. Leonardo is very different from the dead soldiers of the underworld, so soft and pliant, so completely his. He moves and moans because Saki wants him to, because of how good Saki is at making him feel good. He is told to reciprocate and he does, bucking his hips to meet his masters; it's by no means graceful, but Saki is sure it will get better with time.
He does not cry anymore. If there is pain in his eyes, Saki does not see it. The only eyes he sees are Yoshi's, dark and bright and loving, clinging to him like he's the only person in the world.
"Perfect," he murmurs, rubbing Leonardo's cock as the pace picks up, watching from what feels like a great distance as the turtle's head bounces lightly off the dojo floor over and over again. "You are perfect, Yoshi." It's the wrong name, but what does it matter? Leonardo knows whom he belongs to no, matter what appellation his master uses.
He suspects Leonardo's about to come and grabs him again, keeping him from release. Saki can feel himself on the edge, too, but refuses to cave first.
"Master?" Leonardo whispers, peering up at him. Sweat and spit are glittering on his blood-slicked mouth, and it's so beautiful Saki wants to kiss it again. So he does, swallowing a surprised little mmph. 
He keeps going, and going, the mutagen in his blood fueling him as he waits for Leonardo to figure it out. To understand the one last thing his master asks of him.
"Please," Leonardo whispers finally, his voice taut with effort. "Master, please...."
Saki lets go, gives a few more practiced strokes, and watches Leonardo fall apart in his arms, panting and shaking. The sight pushes Saki over the edge as well and he comes, marking his chunin from the inside.
Leonardo can take off his black mask, remove his gear, even burn off any tattoos Saki might give him...but he can never get rid of this, not really. He will always belong more to Saki than to Yoshi, be his in a way his father will never truly grasp, and he will always know it.
He'll never leave. Not really, not the way Yoshi did. And that is good. 
The thought sends a wave of relief through Saki as he pulls out and slumps by Leonardo's side, wrapping an arm around his plastron. It's almost tender, as tender as Oroku Saki has been for a very long time.
They lie for a while, panting, the scents of sex and death and vomit mingling around them. Then Saki carefully scoops Leonardo up, not thinking about his chunin's slight hiss of pain, and carries him off to the bathroom.
He sits Leonardo in the tub and orders him to scrub the blood and cum out from between his legs. The boy obeys mechanically, his gaze dull and distant, and allows Saki to examine him for damage. It's not that bad--most of the blood appears to be from the dead men, although Saki still thinks he should use normal lubricant next time.
"I will give you a few days to heal up," Saki says, nodding thoughtfully. "We'll begin this part of your training in earnest then." He lifts Leonardo's chin once more, reaching up to rub the blood off his mouth. "I hope you understand that this matter will stay strictly between us?"
Leonardo nods. It's not as good as a verbal response, but Saki can let it go. He's feeling generous.
He kisses Leonardo again; the boy doesn't kiss back or pull away, just sits there. Saki grabs a rag and starts scrubbing the mess off his plastron, movements sharp and efficient.
"You did very well tonight, chunin," he says. "I am very impressed." The words cause Leonardo's eyes to brighten, as Saki expected, and he leans into the touch just a little bit more.
He reminds himself to ring for someone to clean the place up and get rid of the bodies later, after the sex-smell has faded. Karai will fuss about the dead men, but she'll remember her place.
Everyone will remember their place, he thinks, caressing Leonardo's chin.