The change may look painful from an outsider's perspective, but to the madman.. it was quite a warm feeling, like shedding one's skin and becoming something so much more beautiful. The Joker, once a human.. if one could still call him so, slowly shifted into that of a large cobra-like creature. He still had his arms and most of his white skin.. scales, even his hood bore a smiling face.. but he was mostly a Cobra, hissing with sharp fangs and a forked tongue.
How he admired how he looked in the mirror and how curious he was to try out his new body.. in the most sadistic way possible. In his defense, he was barely fed in Arkham to begin with, so in a way this was the Orderlies fault for not providing poor little him with anything besides scraps.
So he tested out his body, slithering around the Asylum. Sensing body heat off and on, all the inmates becoming that of hybrid beasts like him, a bit of a shame since he so wanted to see what human tasted like to a snake..
But of course, he became lucky, while the other became very unlucky.. or lucky indeed. 
The boy, having hid best he could under a cart to avoid the chaos, he had taken this job on desperation and, well, he regretted it and agreed with himself that the pay was not worth it at all. When all was quiet and the green water that sprayed from the sprinkler system ceased, did he slowly move his way out.. well until suddenly a white.. tail(?) with purple diamond shaped spots quickly wrapped around his leg that was out and with a yelp of pain at the sudden yank, he was pulled quickly towards something he didn't ever think he'd see.. or want to see for that matter-
There was a very big cobra-like person attached to the long tail that held him and he struggled all to quickly to get away, only to be let go and just grabbed again in a game that made the cobra man laugh, the laugh all to familiar.. The Joker.
"Well well, not a rodent I see, but much better." He wrapped his long snake half around the boy and squeezed, cutting off the human's air and causing a gasp and whimper before loosening his coils to let the boy gasp for breath. "Now now, I don't want you dead for this, oh no.." His tongue slid out, tasting at the air and the boy.. oh my, the boy was indeed in the right place at the wrong time. "Hmm and you work here. Never seen you around before. If I did, well, I would've indeed just crushed you until you popped like a little cherry."
"N-" He breathes in slowly, more afraid that if he breathed to quickly, the snake would cut off his air again. "I just started here a month ago, I usually j-just stay near the office, sorting papers and all that." He explained. 
The cobra ran his fingers through his green tuft of hair. "Well, you are one lucky fellow. You come here at the best time. You're like a little gift."
The boy looked at the Joker with fearful curiosity.
"I intend to swallow you up, whole and very much alive. Just to see if I can"
And his heart sank and did he really squirm than, much to the naga's enjoyment as he watched the human squirm. 
"Y-You can't eat me" He protested.
"And why not?"
"I uh.. I taste bad, yes. A-And I am much to big to fit in your mouth"
"Hmm your smell on my tongue says otherwise. And this is why I'm testing a theory."
"W-Well can't you test it out on someone you d-don't like. We barely met, sir.."
"And if I did that, I may not find them, they may all be animals and I want to try out a human meal before I can look for them. This new body craves food and look at what I have, a delicious looking human. You almost look like you made yourself just for me"
He began to tear up. "M-Mister Joker, please.. please I beg you, I-I will go find someone for you. Someone better I swear"
"Aww tears." His tongue slides out and rubs along the boy's cheek who grimaced while he tasted the tears mixed with the human skin.. it was mouth watering. "And who knows, you might enjoy it. This is what the kids are calling vore right and are into? I bet you are loving this secretly. Who wouldn't want to find their rightful place inside of a belly of something truly predatory?"
"Me! I wouldn't!" He was a mix of disgust and fear and the worst humilating thing is, Joker had a point. Now that he was in a predator's grasp, his own primal mind seemed to somehow know he was caught and it made him fight and wiggle to get out of the coils that tightened, almost playfully around him. Fear gripping his very soul, but also the instinct that he was going to be eaten was there and he let the tears fall and scream. "Help! Someone please!" Better them then him! But no one was around.. no one stupid enough stuck around at least. If anyone had been near, they saw the cobra and the boy, and left him to his fate.
"Hmm I wonder if I can hypnotise my prey like that one snake from that movie. Trust in me... what was your name? Ah, it don't matter." Joker just snickered and with the boy coiled up, he did frown best he could at the fact the boy was dressed and he didn't like the idea of clothing shielding him from the buffet. "Strip."
"Wh-" That took him by surprise.
"Strip or I'll strip you myself"
"I'm not go- Hey!!" He cried in alarm as he was uncoiled but he came face to face with sharp fangs that barely missed him and tore off his uniform shirt and the hands shredded his pants with sharp green nails, leaving him very naked besides his shoes and socks and red in the face. He met the red eyed gaze and just stared in defeat as he slowly slipped off his shoes and socks. "Please.. let me go" He gave another pitiful plea and shivered as the cold coils wrapped him up again, forcing his arms to be at his sides.
The Joker loved it, the pitiful look.. and the acceptance at the fact that he was Joker's dinner. And how he wanted to savor this before Batman came to ruin his fun. "Hmm I think you are enjoying this. Being what you were born to be, you think this was random? You were born to be my food, so be good food and wiggle and cry for me"
And he did just that, wiggled and squirmed in the coils and continued to beg the snake who cared less for him. He was food and he was thinking the same thought. Of Batman showing up to save the day.. any minute now he hoped. But at the same time he wouldn't exactly want Batman to see him naked... and.. well to more degrading facts.. Joker was right about one thing. The idea of sliding down the naga's throat scared him near to death.. but also, it did arouse him some, but not enough for it to be revealed..
The cobra wasted little time, the widening of his jaws hurt at first, having never done this before. He drew closer to the human's head, fangs running through his hair and over his ear, tongue sliding out and under his chin. And the squirming and cries started to intensify than, not that it'd save the boy. He pulled the boy's body up, forcing his shaking head into his mouth. The flavors were so new, yet so perfect. Oh he was going to enjoy this terrified meal now. The Bat will have to kill him in order to pry this from him at this point. The screams were muffled as he closed his mouth, sealing the boy in the dark for a moment. 
The boy's screams were muffled by the warm mouth of the cobra, what ever light that was in the hall was quickly shut off to him as Joker closed his long maw over him, his hair getting soaked from the sticky saliva, the fangs grazing him every once in awhile, causing him to whimper at the stinging pain from them.. he was pushed further into the mouth, until a sudden pressure pushed him further, realizing he was indeed being swallowed. There was no way this was happening. He tried to grab at the suit of the Joker to cease him being eaten any further but his hands were playfully slapped away by the Joker's free hand. Another swallow and his neck slid slowly down the throat.
He was making quite a nice shape in Joker's throat. It was filling him with confidence that he could devour a man. And my did he now know what he was missing, he should have thought of this more often. A fun little sadistic torture followed by a full stomach that wiggled, oh the wiggling was pleasurable to him. The shoulders were a tough bite to eat, and of course it gave the boy hope before he shattered that when he forced his mouth to stretch, ignoring the pain it caused, he wasn't going to give up so easily.. plan be was eating him piece by piece, but of course that wouldn't be as fun as what he was doing now. The muffled screams sending slight vibration as he worked down the boy's boy, forcing the boy down onto his knees from the added weight and making it easier for him to eat. The legs kicking at him barely was felt. Another gulp, another couple inches disappeared.
Eyes open to see darkness, his ears hearing the heartbeat and some other noises he never thought he would ever hear as more of himself joined in the darkness, being pushed down further and further, his morbid thought was how he looked like from the outside, was he making a human-shaped bulge or was he even noticable? Would anyone see what the Joker had done and get him out? He wiggled still, struggling to grab at the jaws but the slippery teeth and saliva didn't give him any grip and his fingers, tickled some by the forked tongue slid down the throat.. half way down.
Half way there and no one tried to be a hero.. seems like it's his lucky night. The wiggling human shaped bulge stretched his suit and was thankful the buttons held though strained as his lively meal squirmed still. Very alive and it didn't show signs of suffocation. Good.
The boy was eaten, still hoping somehow some hero would come and kick this snake's ass and help him out. Anyone at this point. But when he felt another swallow and his waist was pushed down the naga's throat, did he slowly begin to think that no help will be coming for him. It was grim and his mind fought the thought that was slowly winning. The air was hot and rotten. The more he disappeared and slid down, the more he began to accept that Joker may be right.. that he was only born for this moment, to be food for a predator.. an impossible thought, but once again, there was the primal thought that lingered in some humans still.. the painful knowledge that they were prey and not a predator as they led themselves to believe. He was also growing tired, his muscles ached, his energy nearly gone. The air and darkness made him tired..
There was a wider opening, much to his dread as he assumed this was either the opening or close to the opening of the cobra's stomach and where his final destination while alive was. His crotch and behind passed the throat and he was pushed more and more into the opening that gave him some more still awful smelling oxygen. His legs kicking about when he felt the tail release them, only for the hands to grab and force them together. "Da-"
Joker was nearly finished. Once the hardest part with the shoulders were done, the rest was simple. Humans lacked fur or anything that could possibly cause trouble, they were perfect for him. Indeed Joker didn't think he was human any more and that humans were now just his prey.. slowly he swallowed the legs up, the feet were a bit of a sad feeling since this was the last he'd taste of this wonderful meal that provided. But oh well, there plenty other meals he couldn't wait to try out.
So with his mouth closing around the cute little toes that wiggled, he gave himself another gulp and sent the human sliding down his body and travelling down towards his long snake-like body. The bulge slowed to a stop and he had never felt so full and so pleasured by a meal that was wiggling in it's final act for self-preservation. He reached and gave the shape a pat. "You were delicious" He burped loudly, the sound echoing off the walls. "Excuse me. You hit the spot." 
The burp sent the air out of the belly that the boy was in and he really struggled desperatly than, full of fear and final hopes tha the was still going to be saved.. but slowly accepting that no one was coming for him.. and he was going to be dead and digested. His final breaths were heard by no one, and with a few final convulsions, he died within the hot belly before the acids began to seep in to break down the new and very large meal.
Joker ate well and slowly coiled around the bulge, feeling it's last movements before death claimed him. The boy really was the best tasting and hit the spot. His belly was overjoyed and it would be nearly thirty minutes before he knew for sure the boy was no more then just meat.
No heroes today. He'd take his time breaking down the meal that was once a human, he made an effort to hide and digest. When Batman did find the snake, he was full of disgust at the sight of the cobra with his head on a not so human shaped bulge. "My my Bats, a little late this time" Another belch escaped his maw and he chuckled. "He was quite a fine meal, I do wonder what a Bat tastes like.." He smiled, enjoying the obvious look of disgust.