It takes a bit of effort to gain full control of Splinter's body. By the time Falco's done, he's got a splitting headache that the yelling and cursing in the back of his skull helps not a wit.
But that's taken care of with the proper herbs, their appearance and location carefully plucked from Splinter's memory. He makes a valiant effort to push Falco towards the poisons, which is surprising. You do understand that we'd both go down together if I let you get away with that, right? he asks his new companion.
Better dead than a slave, Splinter growls back.
Well, that's rather dramatic, Falco says, sipping some water to get the bitter taste of herbs out of his mouth. And I believe the more accurate word is "partnership." I only want what's best for you.
Many would-be masters have wanted what's best for their prey.
Falco rolls his--their--eyes and settles back onto the bed with a sigh, glancing around Splinter's room. You know, this place is far too spartan for you. With skills like yours, you could have stocked it with the contents of a dozen museums. He stretches Splinter's bad leg with a groan. And gotten some proper meds for this while you were at it.
If you wish to leave this place, do so, Splinter spits. But I am not going anywhere with you.
Falco raises an eyebrow. Funny. I'm making myself nice and comfy in here, my friend. We'll be tied together for a while. Which reminds me.... 
He gets up and sashays over to Splinter's closet, digging through the mass of old heirlooms tucked away there. He doesn't know where anything is, but Splinter does. and that's all Falco needs. "Where is it where is it..." He digs out the old, rusty-edged mirror with a grin. "That's it."
What are you doing? Splinter demands as he props it against the wall, rage halting slightly to make room for confusion.
Checking out my new residence, of course. If we're going to be such close friends, we ought to get to know one another. He dusts off the surface, humming as he gets a good look at the body he's only glimpsed through the eyes of his other rats.
Very nice, he says happily, stroking the whiskers. Even, sensitive, and well-groomed. You have no idea how hard it is to remind the rats to keep theirs clean. He moves on the beard. And I like this, too. It may be a mark of filthy human customs, but it adds to the nobility of the profile.
Falco tries the eyes, watching them flicker from red to black. Quite a cunning disguise. Splinter growls.
He flexes the wrists, enjoying the curve of the bicep. Then he tries a few kicks and punches, leaping back and forth in front of the mirror. Such grace, such strength. You really are a wonder, Splinter. The mutagen blessed you.
I worked for years to earn this strength, Splinter growls. The mutagen did not give it to me. And you do not deserve to play with it like a toy--what are you doing?!
Hmm? Falco's only half listening as he works open the belt of Splinter's robe. Exploring you, silly. A wonder means nothing if it is not savored in full. The robe falls before Splinter can finish his outraged shriek.
The body underneath is impressive, to say the least. Splinter's fur is thinner under the robe, giving Falco an extremely pleasant view of the muscles rippling up his legs and across his torso. The roared threats somehow...intensify the experience, in a way.
You know, he muses, patting his own chest, I never had a six-pack, not in my old life or the new. His eyes flicker down between his legs. And that is definitely an upgrade. His stomach warms with interest at the sight.
Filthy, perverted beast, Splinter growls. Falco just smirks at his reflection as he slowly, painstakingly lets his hand wander down to Splinter's cock. He sighs gently as he gathers the dark, heavy weight in his hand--God, it feels nice to be attached to such a thing.
Get your hands off me! Splinter must be nearly apoplectic by now, as if Falco can't smell the sweet terror under his rage. What is the point of this?!
The point is that we both could use a bit of distraction before the next stage of our plan, Falco says, running a thoughtful finger along Splinter's length. It doesn't stir--the rat must be still trying to fight him. Or maybe, Falco thinks, they could both use a bit of inspiration.
Splinter's still ranting. Your plan, not ours, you--
Tell me, my friend, when was the last time you came from anything other than this lovely paw of ours?
Splinter's mind skitters to a halt at that, but Falco has already seen the answer rising from his subconscious and pulls it into the light. Stop that! Splinter cries, but Falco's getting better at blocking him out by the second.
A night with his lovely Tang Shen...ooh, an awkward night with his lovely Tang Shen. They'd been fighting recently, stressed out by money and parenthood and Saki--it was lukewarm at best and after being briefly interrupted by Miwa's crying, it got worse. They'd barely made it through without getting in another argument.
Falco laughs. That was what sustained you for fifteen years? That tepid memory?
Love was what sustained me for fifteen years, Splinter hisses. With all its ups and downs. I don't expect a creature like you to understand that.
Love? Falco shakes his head. I know what love is, my friend. I feel it for you, and for my rats, and all the creatures we will make together. If I did not love you, I would not want to fill this deficit in your life.
He feels Splinter's consciousness stiffen at that. Speaking of stiffening, Falco's discovered the right tempo in Splinter's memories and his efforts are beginning to show a bit of fruit.
Although I have to admit, my motivations are not without altruism. His last sexual encounter was several months ago--he pulls the memory of it up for Splinter to see. A teenage boy, no older than six, with tight jeans and expensive sneakers. He'd leant against the carder  with an expression of undisguised boredom.
Splinter gasps. A child? How could you?
I paid him well, Falco mutters. But I'm far above such things now. Why go fishing when I have a lovely catch right at my feet?
For a heartbeat he feels Splinter's anger, fear, and humiliation vanish under the weight of blank shock. No.
Oh, yes. The only question is where we begin to indulge.
He reaches into Splinter's mind and digs out an image of April O'Neil. Perhaps this lovely young thing? We could get her alone, ask her to show her gratitude for us being kind enough to take her in. Splinter shrieks in rage and to be honest, Victor agrees. I understand, my friend. She isn't really my speed. Too many curves at her age.
His mind flicks to the pictures of Splinter’s sons. Compared to the few scattered images of April, there's a whole library worth of memories here: the boys training, eating, playing, bathing. Their chubby child's bodies growing into the muscular shapes of young men, pulsing with life.
Look at that, Falco purrs, hand quickening on Splinter’s cock. So strong, so grown-up...and so pure at the same time. Cleans. Untouched. Sealed away from the world like princes in a tower. He shakes his head. There aren’t many children like that these days, let me tell you. They’re all so grown-up and cynical.
You will not. Touch. My boys. Splinter’s voice is trembling.
Why not? Haven’t you earned that right? You fed them, raised them, wiped their asses...this is the least they can do in return.
They do not need to be grateful for what any decent human being would have done!
Really? Most people I know would have drowned those things out of mercy. But you….you saw your potential. You cultivated it well, and let me tell you, those are some very charming fruits.
He zeroes in an image of Raphael bending over. Mmm, quite lovely. You know, you could have built yourself a little harem down here. They never would have thought there was anything wrong with Daddy fucking them on the regular.
Splinter sucks in a breath. I could never--
You turned them into soldiers, didn’t you? Are whores so great a leap? Splinter’s cock stirs under his hand, but the poor rat’s too busy stuttering to notice.
Falco licks his lips. And speaking of soldiers.... He pulls up an image of Leonardo on his knees, gazing up with an expression that's not too far from worshipful. Your favorite--shhh, don't protest, it's all right. And he'd do anything for you, wouldn't he? Absolutely anything.
With a thought he alters the image, splashing Leonardo's face and chest with glistening white cum. He licks his lips, lovely blue eyes glittering with pleasure. "Thank you, Sensei," Falco makes him say.
Splinter chokes. Falco snickers. Sweet, isn't it? We could tell him it's a training exercise, meant to improve his loyalty, his endurance....stamina. The image changes to Leonardo being fucked against a tree, moaning and trembling like a slut.
He's probably been fantasizing about it for years. His big strong daddy, taking him down, dominating him, showing him what pleasure means. He'd love you more than ever.
He wants nothing like that, you monster!
Really? Technically, I'm not even the one getting hard. He looks directly into the mirror, giving Splinter a good look at the cock swelling in his hand.
It means nothing!
Perhaps, perhaps not. Falco shrugs. On to the next contestant, shall we? 
No--
But Falco's already switched to a memory of Raphael. Arrogant, sneering, insolent. Playing at being grown-up, as if his innocence still doesn't manage to seep through every pore. He's a handful, isn't he? In need of a bit of discipline.
He projects Raphael over Splinter's knee, whimpering and yelping as he receives a nice hard spanking. The pert little bottom that had caught Falco's eye before is redder than his mask, jiggling slightly with every strike.
"You like this, do you not?" Imaginary-Splinter asks. "My little whore." Raphael shakes his head frantically, but he can't lie--his cock is out and dripping, and so is Splinter's.
Falco moans, abruptly pushing the scenario forward to Raphael on his hands and knees on the bed, bruised and unsatisfied, shaking as Splinter tends to his sore bottom. "Shhh..." Imaginary-Splinter purrs, slicking up his fingers and gently pushing them inside Raphael's hole, triggering a soft groan. "That's it, little one."
They are my sons! Splinter roars.
They are whatever you want them to be, Falco sighs, pushing the image away so he can have a better image of himself in the mirror. You know, you're quite pleasant to look at when you're erect.
You are a wretched defiler and I will see you dead.
Stop it, you're making me blush. Falco draws a picture of Donatello, blushing and looking around nervously as Splinter ties his hands behind his back. We could dress it up as an experiment, you know. You usually don't give two shits about his work, he'd be overjoyed at the change.
That is not--
Hush. It's all right. We're making up for it now, aren't we? Perhaps we'll even have him make some toys. He imagines Donatello writhing on Splinter's sheets, sweaty and groaning. A spreader bar holds his legs apart so the large, buzzing vibrator stuffed into his plush hole is exposed to admiration.
He might resist, at first, especially considering his lust for that little wench. Falco has Donatello's face go slack with terror as Splinter looms over him, and is not ashamed to admit that it makes him sigh, forcing himself to slow his pace so he doesn't come too soon.
But we'd help him understand, wouldn't we? Donatello's expression melts into desperate pleasure, hips jerking with frantic need.
Enough! No more, I...I beg you.
Hush, my friend. This is for you as well as me. I want you to enjoy yourself. Now it's little Michelangelo slipping through the bedroom door, head bowed and hands behind his back, eyes sparkling with mischief.
He likes games, doesn't he? We can offer him a very nice one. He has Michelangelo riding Splinter, thighs pulsing with effort and head thrown back. Imaginary-Splinter reaches up and grabs his mask tails. "Look at me," he growls, and Michelangelo does, letting them see the drool at the corner of his mouth.
Such chubby cheeks, Falco muses. As if they're begging to swallow...Michelangelo on his knees, mouth open wide to take all of Splinter's cock. His nostrils are frantically flaring for air, but he still sucks away with ravenous hunger.
Mmm... Falco sighs, letting Splinter feel the full brunt of his--their--pleasure. Drops of precome forms at the tips of Splinter's cock. Excellent. I knew you'd come around sooner or later.
You are insane! I do not want any of this!
Of course you do! I'm going to help you understand that, and then we're going to do the same for your pretty little sluts. And once they're nice and trained, well...the sky's the limit, my friend.
One last series of pictures, set in the dojo. Splinter's sons fingering themselves open. They bow when he enters, bending over one by one so he can check that their holes are nice and slick.
Stop it stop it stop it--
Enough, Splinter. All you have to do is watch and enjoy.
It's easy to take the hallowed rhythms of the dojo and pervert them into such more fun. "Katas” where he helps them bend and twist themselves into increasingly more appealing positions, leaving them sweetly vulnerable to his touch as he makes them thrust their hips through empty air. "Meditations”' where he has them sit cross legged as he takes turns sliding his cock down their eager throats. 
And of course, "sparring." Two of them sitting on either side of him, playing with his cock, watching the other two wrestle-fuck. The one who ends up on bottom at the moment of climax is penciled in for a spanking later, while the one who wins gets to make his slow, sticky way towards where his father is waiting.
In this fantasy he decides to have Imaginary-Leonardo walking towards him, leaving Imaginary-Raphael on the ground, while Imaginary-Donatello and Imaginary-Michelangelo discreetly lift their hands off Imaginary-Splinter's hard, leaking cock.
Leonardo settles into his lap and groans as Splinter starts to move, clutching at his shoulders. The others pressing in around them, touching each other, rubbing Splinter's back, faces slack with need. Broken, tamed, trained, exactly where they belong.
Stop, Splinter wails, but Falco's too lost in the fantasy, hand speeding up, the Leonardo in his mind letting out a shriek as he finally--
He comes with a groan, white fluids splattering the mirror as Splinter groans inside his head. His companion is caught in the pleasure, Falco can feel it, and that makes it all so much sweeter.
Falco turns away from the mirror and totters back to the bed on shaky legs, settling back with a groan. He tilts his head back, relishing the afterglow, barely noticing the weeping in the back of his mind.
Lovely, Falco says. If this is what imagination is like I can hardly dare to dream about the reality.
Silence from Splinter. Then, words like brittle shards of ice: I will kill them before I let you hurt them that way.
Will you? Falco asks.
If I must.
That's interesting, my friend. Very interesting indeed. Falco sits up, grabbing a rag to clear himself before pulling the robe back on. You have a bit of ruthlessness at your core, yes? I chose well.
Go to hell.
Already been. Didn't suit me. Falco stands up and smooths himself down, sparing one last glance at himself in the mirror. For a second--just a second--he sees the expression there with a hate as dark and fierce as a night storm. Then a second later, it's his own triumphant gaze.
Something you want to say? he asks Splinter, trying to hide how the sight caught him just a little off guard. But the rat is silent.
Assuming it must have been a trick of the light, Falco shrugs to himself and grabs the walking stick. He hums jauntily as he makes out the door.