This post is an entry in the 38th Finish the Story contest, hosted by the remarkable
Join the fun and write your own ending!
The prompt is interesting. A subject is being experimented on by cold, inhuman beings. Where is he? Is he a he? How can he escape? Will he escape or succumb to madness and mutilation?
With 500 words being the limit, the nature of the escape is going to be limited as well. No novel-long method of a slow escape from the castle by using a toy-sized replica of the castle to weave a yarn from which you can weave into a rope in order to descend down from the tower It'll have to be simpler then that.
The Taste of Chicken
by 
"What do you feel now?" The scalpel of a monotonous voice, cold as the halogen light blinding him.
"Let me go".
A sudden electric shock followed and almost broke the arched vertebrae of the specimen n. 19-B, while penetrating his limbs.
"What do you feel?"
"The ... the taste of a chicken".
Dense whispering, silent annotations, white everywhere.
He was floating in that white, for hours, perhaps days. Subtle lines, at the corners of his eyes. The last bulwark of Euclidean geometries. Over him, the ceiling was like an enormous virus. Not alive, not dead. Up there, all the gluttony of a pulsating white blasphemy was unfolding over his impotent being. A dodecaphony, ever hungry of new semitones in the musical scale of his moribund ego.
He felt his soul's entrails slowly peeled like an onion. That non-color was slipping inside him, like a sickly glucose sludge, inside every cavity, along every neuron, hair, capillary. It was a profound evil, different from pain. Pain is a vowel. If you're good enough, you can observe it from the outside of yourself till you inevitably lose consciousness.
That, instead. That was nothing less than a grinding profanation of his soul.
"Now. What do you feel?"
"Whaaat do you feeel ..." was the mumbled answer, the sound resembling the broken lung of a deflating accordion.
Silent annotations following.
The synaptic stimulation was proceeding well, soon they could present the product to Mother Unit. It was said that, in prehistoric times, the human being populated the nano-swarm, when it was still called Earth. According to certain niche schools of thought, this.. thing.. could have been at the top of the food-chain. Go figure.
A new product, very efficient in its own way and not missing that pleasant touch of chaoticness, this human. No doubt that the Mother Unit would have liked it and find it entertaining, if not even useful.
The chief demiurge gently closed the skullcap of the specimen n. 19-B and left the room with its cohort of servile apprentices in tow. Enough for today.
My Ending
Enough for today. Enough for a lifetime. What was needed was respite. Respite from the prodding of the mind, the prodding of his soul. Respite from existence. But that later part could wait. Had to. First things first. 19-B needed to get the hell out of that tesseract. And that’s exactly what he’d be doing with his day.
Four thousand, six-hundred and seventy-one times. It took that many experiments, procedures and otherwise unconscionable forays into his mind before the demiurge or his sycophantic scientists made a mishap. It was all he could do not to smile when they made their way out in tow.
One of the followers left a surgical beam on the tray beside him, just in reach of his hands. He was ready to grasp it and free himself when the fear struck him. Think you can do it? Think you can get out of here? You leave this room and what? You’re just gonna walk out the front door?
“No,” he said out loud. “I got out of that fucking sand whale, I’ll get out of this.”
And if they catch you? You really ready for another four thousand proddings? Cause these wont be reserved. No. They’ll have malice behind them.
“I can deal with that.”
19-B stretched his fingers over the cold tube, sliding it gently into his fingers. He twirled the surgical beam like he’d done a pencil as a kid, back in elementary school. Back in the time, before those nano fuckers turned everything grey. He steadied it and shot that bone-cutting laser onto the steel cuffs and melted them through, freeing his hand and then his other and then his feet.
He fell to the ground, the sense of gravity refreshing and crushing all at once. The atrophied muscles ached and it took him a number of tries to acclimate himself to the odd and arduous trial that was walking.
Told you. A cripple ain’t going anywhere.
“I’ve had enough of these little voices,” 19-B said. “Shut up.”
Can’t. I’m a part of you now. You can end me, you know. You know. You know how.
“Not yet.” 19-B stood up, breathing through the fire that pulsated in his thighs and ankles and back. “I’ve got a lot of shit to do today.”
He smashed his fist on a panel and the door slid open. Stepping out into a hall, bright halogen lights extending off in both directions.
Doors. So many doors. “I wonder…”
Behind those.
“Are there…”
People like us?
“Like me.”
19-B stepped forward to the room across from him. On a holographic display beside the door was the title, “19-A”.
19-B gulped, realizing for the first time just how thirsty he was.
You’re starting to get it, kid. Starting to figure it all out.
“No.”
Open the door. All of them. Let’s have a party.
“First time you’ve said something I agree with. Time to get those sick fucks.”