It was a late night in the royal laboratory and Gaster was working hard on his research for Determination, trying to understand and figure out what makes it so powerful and yet dangerous for the monsters of the underground. He studied the samples that he has to get as much information that he could, moving station to station, looking at graphs that he made and getting physically annoyed at the lack of solid answers that he is getting. As the time moves on the more annoyed he gets with papers everywhere and the deadline getting to a close, and just wanting something solid for the king but with his growing hunger making it harder to concentrate Gaster started to go mad and began to throw things around, mumbling “I need to know, i need to know, i need to know”. Gaster took one ditch efforts to fully figure out Determination and grabbed the vial of concentrated Determination he extracted from the bodies of the humans that came before. Gaster quickly found a vein then stabbed the needle deep into the vessel and injected himself with the Determination. It quickly hit his body making him fall to the floor, and with heavy breathing he attempted to resist it from killing him till the severe pain went across his entire body making him wish for death, but he dealt with the pain and calmly breathed, and with sheer will-power and his own type of determination the actual Determination passed and settled into his body but without making some changes to him. His hunger turned into a more vore-ish hunger his ass and balls became hungry, more hungry than his actual stomach. His mind snapped in all the places and thoughts of things he would never have wanted to do before. 

A sound of the lab door opening broke his thoughts, turning around to see the King and Queen in the doorway.

“Gaster my man! How the research going on Determination?”

“Heh... Why don’t i show you ‘Sir’”

Gaster stood up with a hungry crazed look in his eyes and with the feeling of his ass and balls ache for a meal, Gaster stared at the king and queen for a bit and looking them down and up with the motion of licking his lips. The King and Queen looked and felt uncomfortable at Gaster wondering why he is staring and licking his lips at them. In one very quick motion Gaster ripped off his clothing revealing that dump truck of a ass and that hung, girthyness of a cock with a pair of watermelons for balls.

Toriel’s face went red at the sight, but it didn’t last long as she got light-headed and her legs got wobbly as she fell backwards onto the floor and passed out. Asgore quickly kneeled down and tried to wake his wife up by waving air onto her face and petting her cheek, but with no success he got extremely angry at Gaster and with a fast standing position, ready to pummel his royal scientist Gaster was already in the motion of falling from the air and sitting on the kings head, and with a quick popping sound the kings head went into Gaster’s sweaty ass. Gaster would give out a sloppy wet moan as his ass opened wide and wider as he pushed and bounced on the king till the king’s shoulders were in his ass to make sure Asgore couldn’t thrash around to get him off. He wiggles and wiggles, bouncing every so often to get the king further in by the second and getting to the bottom of the chest. His ass at this point being so stretched it felt like Gaster could split in half any second, but only thing that did happen was Gaster getting a massive leaky boner and the thrill of this.

“My king~ You will make my ass so huge when are are digested~”

With the sheer weight of Gaster and Asgore’s legs becoming weak from the constraint of the ass the king fell onto his ass to the floor, trying now to remove his royal scientist off him with pulling his arms against the ass he is trapped and being sucked into. Asgore noticed it only made the walls of the ass more lubricated and making it easier for Gaster to slide down and give louder wet moans. The Determination he injected into his body made him have more of an elastic skin and bones, so Gaster can shove any size body no matter how fat and tall they were. Gaster bounced and wiggled seeing the Kings face and hands pressing against his stomach. The facial expression on Asgore gave Gaster more lust with that face of terror in the kings belly mold. Nearly having the entire upper body of the king inside his belly and ass Gaster would stand up feeling the King kick the air Gaster would look at his research while his ass did the rest of the work and gobbled up the legs then feet. 

“This D-Determination is amazing! I need more of it... But where to get more”

Gaster stood at his messy desk thinking for a while till he remembered the King and Queen has a adopted human son that has the Determination that he can take. Gaster then turned around to start going for Chara, but saw the Queen moving and knew that she needed to be dealt with, so he stood over her and waited for her to wake up. It didn’t take too long for Toriel to come back to consciousness and seeing Gaster over her and not seeing her husband.

“W-where is Asgore?”

“The King is somewhere and you be with him soon”

Gaster would grab her feet and shove them into his sloppy wet asshole while the queen gave a disgust look at Gaster from the loud squishy sounds. Gaster would loll his tongue out with the feeling of his ass being stretched once again. Toriel tried to crawl away after the gross shock faded from her but with her feeling loopy from being passed out and hitting her head on the floor, she really couldn’t do too much but hope her strength comes back before it's too late for her. By the seconds she started to think her strength wasn’t gonna be back as her entire legs to her fat crotch was fully into his ass, she even moved them around inside and felt her husbands body inside, and it hit her hard. 

“W-Wait is my love inside YOU!!??”

Gaster just smiles in lust finding out her body will be a challenge to stuff inside him even as Asgore had filled him up quite fully and just her legs was enough to fill him up more. With some heavy breathing and his determination he pushed onwards and pushed his fat ass down, biting his lips and grunting hard he managed to get the width of her ass and hips in, he stopped and sighed in pleasure his cock started to burst couple ropes of hot and smelly cum all over his lab. Toriel feeling the pressure of his tight ass slowly sucking her in but with no energy she couldn’t do much, it also felt like her energy was being sucked out of her and into him. Moments later Gaster got his composure back and went back to pushing down and getting more of the Queen inside while his belly expanded outwards more and his ass widens as well. Toriel felt his ass pulse and throb, slowly sucking her in as Gaster kept cumming, this sensation was heavenly to him the sheer amount of pressure and fullness of his body made him feel sensational at best. Gaster felt the Queen start to move more as she got a bit more energy back, so Gaster moved quickly sucking and pushing her in, at this point half her upper body was in, and he started to look like a blimp. He sucked her in more and more hitting her massive breasts. He stood there now having the dilemma of how to get her freakishly large breasts to get inside his body, he thought for a while then decided to leap high into the air and slam onto the floor. In the process Toriel screamed in pain, but her breasts popped inside having only her head and arms out Gaster reached around and pushed her head in then the ass slurped up her arms as both King and Queen are now inside his body to be ready to get digested.

In Chara’s and Asriel’s room, Chara wakes up from the scream and loud thud from the basement so Chara looks over to Asriel’s bed seeing his brother sleeping like a log and sighs from relief, but the sounds got him a bit worried so he got out of bed and cracked open the door to peak out.

“Mom?... Dad?... Everything okay?”

Chara looked left, down the hall towards his parents room then looked right, down the hall towards the rest of the house. Chara would open the door the rest of the way and walked out towards the main room to the basement stairs, he would hear more sounds coming from the basement so he ran to the kitchen and grabbed a sharp knife, coming back to the stairs he would slowly walk down and looking over the edge to the bottom.

“Mom?... Dad?... Is everything okay down there. What’s those sounds?”

Chara took the last step to the basement floor and looked down the hallway seeing the door that is always closed and always is locked, being wide open and a shadow moving inside the room. Chara slowly walked towards the door and when he got there he peaked in seeing nothing but a huge mess inside. He took a few steps inside and the door slammed shut Chara spun around seeing Gaster in front the door. Chara looked mortified seeing Gaster looking like that and seeing his parents terrified faces pressed out of his stomach. Chara lifted the knife and grinned than ran towards Gaster knowing that he will hit, but instead Gaster grabbed Chara by the neck and lifted him open.

“Your Determination will make me better”

After saying that Gaster grabs his huge cock and pulled his crown to be aiming at Chara’s legs then he lowered Chara down and shoved his feet in the slimy tunnel. Chara would swing the knife at him only for Gaster to slap the knife out of his prey’s hand and kept pushing him into his urethra. This gave Gaster a stinging pain sensation and lolled his tongue out again and throwing his head back, feeling his urethra stretch and bulge with Chara’s legs. Chara panicked in Gaster’s grasp the knowledge of his entire legs are confined inside the shaft made Chara uneasy just thrashing to escape from the deranged lunatic that was shoving him down their cock. Gaster used his free hand and slowly stroked his cock the stinging sensation made it more mind-numbingly great and the bulge of Chara only got bigger as more of his body slid right in.

“I can tell y-your Determination will make me so happy, Chara”

“Gah! Let me out you freak!”

Gaster wraps his hand-over Chara’s mouth tightly to shut him up, pushing Chara deeper and deeper his belly then chest disappeared into the shaft. The urethral stretching sting was devine, and he stroked faster but now with two hands till the rest of Chara disappeared within the shaft. Chara’s feet and bottom legs popped into Gaster’s balls. Chara would feel the shaft move like its breathing, tightening around him and sucking him down and in mere seconds his ass dropped into the cum filled sack, then his belly and chest till his head joined in. Now that Gaster got all three where he wanted all he did was sit on the labs sofa and rested, enjoying it as he let the three to begin digesting.

Gaster sighs in relief from hearing the gurgles and churning of his ex King and Queen and their son. They all moved in discomfort as they felt their prisons heat up and tighten, the King and Queen started to get itchy as their skin starts to feel sensitive. The stomach acids coated their skin and soaking in. Asgore moved frantically and screamed looked over to his wife with half his face being exposed skull. Since he has been in the longest the digestion process has been doing its thing. Toriel looked mortified and begged to be let out but after mere seconds of no answer she already knew she be getting digested with her husband. Gaster lets out a loud belched and patted his stomach while resting on his couch and slowly jerking off, his cock throbbing and his balls tightening and loosening. Chara was pushing at the walls of the balls, Gaster smirks seeing the hands pressed firmly against his flesh feeling the human wiggle and move, not knowing what way is up or down. Chara pants from exhaustion and a sharp pain hits his body. It was so dark in there he couldn’t make out what was what but he felt his skin slowly solidified and thicken like goo. His sweat become goo, his fingers clumping off him one by one. He couldn’t wiggle his toes anymore or even feel them in that matter. Chara would push again even for sounds of splashing to be hard and moved his hands to his face, feeling gooey nubs on his wet cheeks. Seconds later the pool of cum raised as Chara felt bit off he tried to move his legs... Nothing happened though, his legs turned to sludge and soon his arms and the rest of his body. Asgore felt so much pain feeling his flesh be slowly melted off. After a while of thrashing in agonizing pain Asgore stopped abruptly and stayed still for minutes on end as the visible bones rotted and turned to mush. Toriel touched her husband and nudged him only for him to crumble into pieces and gets submerged into the acids. Toriel screams in horror and tries to push her way out, in every direction she could but the way she came in was sealed tightly. Toriel groaned and panicked the heat of Gasters insides only got hotter by the second, her blood felt like it was boiling and her body got skinnier as her insides were being digested from the acids that were soaking into her body.

“LET ME OUT!!! LET ME OUT GASTER!!!”

Toriel started to cry, the sight of her husband being melted away and the pain she was having from all this was too much for her... As time went on she gave up, she felt her skin peeling, her exposed bones turning to mush. She just huddled into herself till she also was submerged into Gasters acids and perished.

After those three got digested Gaster sat up from the couch with a big smile and with a couple seconds passing his cock and balls grew double in size, his balls feeling heavier than before, his cum slushing around with every little movement he did and his crown leaking pre-cum like a fountain. As he was very pleased by the results he felt his ass wiggle then bounce as it grew with each cheek triple their original size. He was in such a glory feeling from the success of the experiment even though he knew they aren't coming back, and he was perfectly fine with that. Thinking to himself “They are better off as my assets fat anyways”. Gaster looked up in the air and saw their souls leaving all happy that at least they can be in the afterlife together but with all this Determination traversing through his body and veins he was able to grab the souls of Asgore, Toriel and Chara looking at the souls with such a malicious happy smile. Gaster would separate Chara ‘s soul from Asgore’s and Toriel’s souls with them looking at each other with terror in their faces then more flushed over them as Gaster moved them to his ass and shoved them inside. The two souls could move around more properly with how much room they have in those fat sweaty cheeks but with souls being more fragile they quickly dissipated fast, making his ass grow even more. Gaster was loving this, the Determination still high in his body he grabbed his massive cock and with the Chara soul in his hand he thrusted his arm down his urethra, stretching his cum tube and let go of the soul. Giving his meat some strokes to help his cock swallow the soul till it disappeared into the balls. Chara’s soul looked around scared not wanting this to happen again it tried to escape from the cum chambers but to no avail the soul slowly melted to more sperm and ball fat. With Chara’s soul gone his balls became heftier, making it harder for Gaster to move. He tried to move for a while to get use to his new assets weights till it popped into his mind that he can teleport slightly then he heard noise coming from upstairs.

“Oh right Asriel the prince is still here... Time to give him a visit”

Has he given a wide toothy smile.... 

{Asriel’s P.O.V} I would yawn and stretch really hard, smacking lips together and scratched my ass as i got out of bed and walked out of my room into the hallway. Walking down the long narrow halls of the house till reaching the grand entrence room, i felt a little unease like someone is watching me from the shadows of the house, i then walked into the living room suspecting my mom to be in her chair next to the fireplace reading. I noticed that she wasnt even there though but the fireplace is lit and with fresh wood kinderling. I walked past though thinking she is in the washroom or something. Finally i made it to the kitchen and went to the cupboards for a cup and grabbed water “Gulp” “Gulp”. That was refreshing i thought and left to go back to bed as i walked through the house i heard weird noises so i looked around frantically getting scared from the weird noises. “Hello? Anyone there”. I then saw glimpses of someone in the corner of my eyes, every second in every corner of my vision. With fear kicking in a bolted away, heading towards my bedroom door. Looking back i saw a creepy guy staring at me down the hall as i got to my bedroom i quickly slammed the door and panted, leaning against the door then turned around. I jumped backwards to the door as i saw that guy from the hallway in front of me in my room. I was worried and wondered how he got in here, a massive tall guy with a really huge fat ass and a cock that could destroy anyones insides.

{Asriel’s P.O.V} We both just stood there for couple minutes... just staring at each other for what felt like thirty minutes till i decided to speak and say something but before i could he grabbed my clothing and ripped them off exposing my body and huge meaty cock. I was gonna say another thing but he grabbed me and pressed my naked body against his then all of a sudden i was feeling my ass stretch open and what could feel like my bones of my ass breaking then he was thrusting roughly inside, i could feel my intestines and organs be moved to the side of my body. The loud wet plaps filled the bedroom as i was forced to bury my face into his sweaty and gross body to not disturb Chara’s sleep. I was panting as this man ruins my ass, i felt powerless to this man all i could do is moan as he does his thing, feeling his hefty giant balls smack my back with every thrust he did. I could hear his heart racing and his quiet moans to not wake up my brother. I quickly felt his base slamming on my fat cheeks, he fucked my internals rougher than at the start, making me bite down on my lips from the pain of it all, his grip got tighter and he was holding me a lot closer like i could merge into his body. All of a sudden i felt pressure build up inside me and a wet flooding sound as i look down seeing a bunch of pre-cum slide down the mans legs and seeing my belly grow. He grabbed my head and shoved my face back into his body, i could taste his sweat and the smell we were making was awful, i wondered how Chara is still asleep by this stench so i moved my blushing face to his bed from over the man’s shoulder and saw a empty bed, now worried the man pinned me to the door and fucking me even faster. I had to wrap me legs around his waist to stay still. After a bit the man moaned loudly said something that shocked and scared me as i felt a burning heat in my stomach, he unleashed a massive load and countless amount of roped cum into me. I couldnt hold too much as i throw up a bunch of the cum, the thick gooey and its salty taste hanged from my mouth. Than he put me down to the ground, my belly made me look so damn pregnant...

*********
Gaster looks down at the Prince seeing him panting and holding his over bloated stomach and with a quick glance to Asriel’s cock, Gaster would loom over his body and crouch down, grabbing the goats cock and lined up then he sat down and moan with his anal canal stretching from the girth of Asriel’s cock and started to ride their cock all the while holding the goat down. Gaster’s fat, sweating ass would constantly bounce and jiggle with each movement, Gaster would loll his tongue out with saliva dripping down onto Asriel’s chest. Asriel tried to hold in his moans from the rape he was experiencing but he also was enjoying Gaster’s fat ass and even held tightly on those sweaty cheeks, giving some nice hard spanks and leaving red marks. Gaster kept his movement to a nice pace, wanting to make the riding last longer as it felt so great on his overly sensitive ass. The Prince unintentionally thrusted against Gasters movement to get deeper inside his warm, lush ass and wanting more. The Determination surprisingly soaked into Asriel from Chara’s cum load thats been digesting in Asriel’s gut, making the goat prince more into this and wanting to blow his load deep into his fathers royal scientist. Gaster pulled his head back in lust, bouncing on Asriel’s lap feeling and hearing the wet plaps of their intercourse and his cock flopping around, smacking Asriel in the face as Asriel tried to move his head out of the way from Gaster’s cock but to no success with that slab of meat the prince is dealing with, his fur being coated by Gasters sweat while Gaster is loving the ride. Asriel couldnt handle to much his balls were acking and needing the release, the prince would give another ass slap and moaned like a girl feeling his balls release a massive load of pre-cum inside, Gaster groaned feeling his belly inflate and wanted the real cum so he clenched his ass together along Asriel’s cock and rode faster, staring right at Asriel.

“Come on prince... fill me up. i need it!~”

Gaster bounced more rough making Asriel groan in needy pleasure as his balls churned till Asriel grabbed Gaster’s hips and slammed those cheeks to his base giving a loud “TAKE IT ALL” the goat would shoot rope after rope of thick creamy cum into Gaster’s ass and in seconds Gaster would cough up cum as his body overfilled with cum for moments to come. After a while Asriel let the royal scientist go and Gaster would fall to the ground panting as they both laid on the wet floor. The laid there for thirty minutes straight in silience as Gastor gains enough energy to sit up and face towards Asriel his eyes widen seeing the princes massive veiny cock throbbing and wanting a round two. Gastor leans and grabs ahold of that massive cock, moving his head next to the meat in his grasp and rubbed his cheek over it giving a few licks here and there. 

“Look at you dear Prince, ready for more~”

Gastor lifted his head to the prince’s tip, shoving that fat crown into his mouth “galk” “gulp” Gastor starts sucking his cock, moaning in lustful heat. Asriel lolls his tongue and quickly grasps onto Gasters head and holds Gastor in place before slamming his face into his crotch. Gastor rolls his eyes back feeling how much his throat is getting stretched and gapped, Asriel’s tip pops into the stomach unleashing a small amount of pre-cum in. Asriel held him there for a few seconds groaning in pleasure and proceeds to use Gastors throat as a fleshlight. “GLK” “GLP” Gastor was slathering that meaty cock with every forceful thrust the Prince was giving, Asriel was enjoying using his families royal scientists throat thrusting so rough in a tired state, smacking his crotch to Gaster’s face.

“You r-really want it? Than t-take it~”

Asriel thrusted faster, gripping onto Gaster’s head tighter and the sound of sloppy slurping sped up, saliva bounced off both of their flesh and flying in different directions. Gastor was looking so pleased, enjoying the Prince’s tasty cock and moaning for more. Asriel started to shake from getting weak he felt like bursting another load but he stopped and panting forcing Gastor to bob his head up and down along the goat’s meaty cock not wanting this oppertunity of gulping down that thick salty cum. Asriel moaned loudly his balls churning rapidly for release as a few more gulps down his balls exploded and sent out a huge rope of cum then a glob of it down Gastor’s throat making him drink it down fast. After Gastor swallowed all of Asriel’s cum he stood up and loomed over the Prince his cum dripping asshole dropping cum onto Asriel’s face as he lows down Asriel starts to try getting his hands pushing against those soft squishy ass cheek till his face was engulfed into the ass.

“Enjoy my ass rince and clean up down their as well~”

Gastor wiggles his ass on Asriel’s head till he was comfortable, feeling that goat tongue slide into his ass made his cock twitch hard so he grabbed his own meaty cock and stroked slowly, moaning softly as the Prince licked deeply in hopes to get Gastor’s ass off him he used his hands to spread those cheeks far to get really in there, but only cause the asshole to dump half a gallon of cum onto Asriel. The cum seeped in between the head and cheeks and onto the floor where it pools more around the both of them. Gastor was smelling the steamy cum that was also mixed with his anal sweat forcing him to get more horny and riled up, stroking even faster. Asriel would gulp down what was poured onto him before he drowned in his thick cum and kept going into that ass before he knew it his head popped into Gastor’s ass causing the royal scientist to squeel and pant.

Gastor bites his lower lip and reached around to grab Asriel and in a quick second shoved the goat’s top half up into his ass and held onto the legs and rode the Prince’s body like a dildo. He lolled his tongue out so far as he used the full lenght of Asriel’s upper body, pounding his ass to the floor and feeling the Prince’s face bulge out of his stomach every second as he stroked more rapidy at his reddening cock. 

“O-oh dear Prince y-you feel so damn amazing. Y-you’re whole body j-just stretching me so, so much~”

Gastor moved his hips faster and his strokes harder, feeling the pure lust of it all flow through him. His cock aching from the lack of breaks but he could care less with all this ectesy, his cock began leaking the globs upon globs of pre-cum with every stroke and Gastor could see the disaster the room is becoming with all the cum stains on every wall, floor, door and ceiling. The muffled sounds of Asriel was starting to be heard as he got hotter inside Gastor. Asriel pushed at his walls with what strength he had and with no avial he couldnt make a budge to get out of that tight, hot and sloppy anus.

“P-please stop i want th-this to end...”

Gastor sighed but smiled and stood up as a smile came upon Asriel’s face but quickly lost it as he felt Gastor popped his ass in and then his legs and feet followed suit. Asriel tbegan panicking and thinking of why this is happening to him as he then felt himself bouncing while Gastor started to twerk on the slimy walls giving pressure. Gastor would twerk on everything first the wall then the bed frames to feel a pole shape between his cheeks. Asriel felt tighter inside those flesh walls his cock so tender from all the sex, he could touch it and he would just start cumming as the insides heats up he felt tingly as the sweat turned to a acid like substence. Asriel panicked even more as his fur was burning away and soon after his flesh started to deteriate. Even tho he was melting his cock twitched, grazing the hot walls and started to cum hard. Gastor was still twerking on the bedframes till a huge shot of cum bursted out of his ass from Asriel as he moaned his own cock shot a huge load of cum onto Chara’s bed from the other side of the bedroom.

“G-good Prince cum for me a-as you become part of my ass fat~”

Gastor kept twerking and massaging his ass to feel Asriel fill him more out while Asriel was becoming goo he looked around defeated to his new home as his face bursted when Gastor squeezed there. As Asriel wakes up seeing himself as a soul and ready to move on he was happy till Gastor grabbed his soul tail and pulled him back down and shoved the soul into his ass. 

“MMM~ Where y-you going? I am not done with you!”

Gastor sits down panting and wiggles his ass on the floor feeling the soul squirm around inside and feeling his enormous ass grow some more, the stretching felt so great the growth made him slightly move along the soaked carpet. Few minutes later the squirming inside faded away and he was all alone now. So Gastor gets up from the floor and walks out to the main room, looking around he went downstairs and to the end of the hall and out the giant gate thinking to himself as the gate slowly closes behind.

“Who next... hm... Perfect lets pay a visit to my kids Sans and Papyrus...”

As he grins with delight the sound of the gate closing behind him ends a fun last day with the Dreemur family...