Tags: underfell, merfolk au, mermaids, canon typical violence, foul language, canon names, suicide mention, injury, permanent injury, surgery, non consensual surgery, medical drugs, dubious consent drug use, recovery, unhealthy relationship, slow burn
Note: Inspired by Sushi Chop Suey by DandelionSea. If you like mermaids, oviposition, or Swapfellcest you should definitely give that fic a look! I personally love the bold choice to have the whole thing without dialogue, it really enhances the experience.
So in light of what this is inspired by and my own favorite tropes the uh… the intended OTP starts off really rough. It's a pretty unhealthy relationship so if that's a hard no for you then I do recommend turning back now. Mind the tags and all that.
Gaster swam along slowly, poking at the rocks and sand along the seafloor. The required guard, this one named Nautilie, trailing along behind him with a bored expression practically etched onto her face. If Gaster had the choice he wouldn't bother with a guard at all, but as their city's only "doctor" Queen Undyne had declared he had to take a guard with him whenever he left the safety of the city. Mostly he just used them as pack animals, it certainly made it possible for him to go out further and longer on his foraging trips before needing to turn back for being overburdened. He was just inspecting whether a conch shell still had the animal inside it or not when something in the corner of his eye socket caught his attention. He turned his head and dropped the shell in surprise.
Bones! Large ones too, unusually clean for so many of them gathered together. Perhaps a carcass a neighboring pod had dumped here while cleaning out their stores? If that were the case there probably wasn't any magic to distill from them. Still, such large bones would be a goldmine if they were untouched by apothecary hands, well worth at least looking at in Gaster's opinion. He swam over to the bones, noting how they were strewn out like an actual body, mostly buried in the recently disturbed silt.
As Gaster checked the remains over for any traps or signs of sea life living within he murmured to himself, "ROUND SKULL, WIDE SHOULDERS, LONG ARMS…"
"What?" Nautilie asked from a respectful distance.
Gaster sighed and looked up at the guard to sign along as he spoke, "I THINK THIS MIGHT BE A HUMAN CARCASS. VERY POTENT. VERY USEFUL."
"Okay, should I pick it up?"
"MAYBE. I MUST TEST FIRST. NO POINT IF ALREADY DRAINED." Gaster went back to ignoring the guard, just as well since she didn't respond anyway, and slowly inched closer to one of the outstretched arms. Amazing the way it was still laid out as if it were alive despite missing all connective tissue, Gaster feared it was simply one of those human skeleton models that had been thrown away for being broken. What a waste of time that would be. He put his hand on the carcass's outstretched fingers.
The carcass snarled and weakly thrashed. Gaster flit back and away from the apparently still living bones, Nautilie dropped her armful of components and pulled a rusted harpoon from its holster on her back.
"back off ya bottom feeders," the unknown monster slurred out even as they slumped back into the silt.
"OH," Gaster said in surprise, he reached out to poke them again, "YOU'RE NOT DEAD AFTER ALL."
"i ain't sea foam yet," the monster mumbled, voice fading out.
Gaster held a hand up to stop Nautilie from nudging them with her harpoon. The monster was clearly on their last fin, so weak Gaster hadn't even sensed any magic radiating from them. Strangest of all they seemed to understand him despite being face down in the sand and unable to see him sign. Had he even signed? Gaster put his hands behind his back to make sure he didn't sign. "WHAT IS THE CAUSE OF THIS? ARE YOU INJURED?"
Nautilie raised a brow at Gaster, clearly confused by this exchange. Thankfully she was one of the quiet guards, hence why Gaster often chose her.
"'m dead iz wut i am, jus' ain't foam yet," the monster mumbled, voice blurring and fading out at the end.
How interesting. Gaster hadn't met another monster that could understand him in years, not since he was a young skeleton and still had a family. He began carefully sweeping the silt and sand away from the injured monster to better inspect them. "HOW SEVERE IS THE INJURY? USUALLY ONLY A STATUS AILMENT CAUSES A SLOW DRAIN ON ONE'S HP."
"'m a cripple, ya jerk. so if yer not gonna put me outta my misery jus' go the fuck away!" They twisted to bite out their words at Gaster, who had to wave Nautilie away again. The stranger wasn't anywhere near close enough to bite Gaster, they had tried to keep their tail still, and rightly so now that Gaster could see it. Still half hidden by a cloud of disturbed silt, Gaster could clearly see there wasn't a single ounce of ecto-flesh summoned, putting all their bones on display. One of the last vertebrae looked to have been crushed, nothing but shards of bone held together by pure mana faintly glowing an angry red. Below that were the thin, delicate bones that formed the caudal fins, or what was left of them. Most were outright missing, the few left either cracked or broken in half.
It looked incredibly painful. Still, the wound alone wasn't life threatening unless left unattended for rot and infection to work their way into the exposed mana. Considering the wound wasn't actively leaking it clearly had had enough time to heal up that much. Which was far too long without any medical attention, in Gaster's expert opinion.
Gaster moved back up to better look the monster's torso over now that they were propped up on their arms to glare at him. "IS YOUR TAIL THE ONLY WOUND? OR DID WHATEVER DID THAT DAMAGE ANYTHING ELSE?"
"wha? why're ya…? what's it matter t'ya? jus' foam me or go away!"
"Wait, you can actually understand him?" Nautilie asked incredulously.
Gaster ignored the monster's confused sputtering as he looked their ribs over. Lots of cracks and fractures, likely from whatever crushed their tail. Probably something with strong jaws. Their ecto-flesh must have been riddled with lacerations, no wonder they just dismissed it all. And the intense pain would explain the suicidal talk, though the fact they aren't dead yet means they haven't tried to kill themself, and that meant they likely weren't ready to die yet. They hadn't even tried to make them do it for him by attacking the guard or Gaster himself. A good sign, if Gaster's going to use them it's better if he doesn't have to keep them on suicide watch the whole time.
"WHAT'S YOUR NAME?"
"why's it even matter?"
Gaster simply stared the monster down, when he asked a question he expected an honest answer and it was best this monster start learning that sooner rather than later.
The monster sighed and seemed to deflate, perhaps too tired to keep barking at Gaster or perhaps thinking he intended to use their name to perform some sort of last rites. "'m sans," they said quietly, voice starting to fade as their eyelids fell to half mast.
Gaster dug around in the little pack he kept on him for emergencies, his fingers brushed against a clam shell, the Intent radiating off it telling him what it contained. He quickly pulled the shell out and opened it, revealing a thick paste inside. He dragged a phalanx through the paste to gather up a generous portion, then held it up to Sans's teeth, his other hand firmly cradling the back of his skull once he put the clam shell away. "HERE, EAT THIS."
"why? izza waste of good meds. if yer gonna foam me then jus' get it over with."
"DO AS I TELL YOU OR STAY HERE AND LINGER IN PAIN UNTIL SOMETHING MORE MERCIFUL COMES ALONG."
Sans rolled his eye pips, barely visible for how faint and fuzzy they were, but obligingly parted his shark-like teeth and let Gaster shove the paste into his mouth. Gaster even caught a glimpse of a barely there red tongue flick out and lick up the medicine before the teeth closed.
Gaster turned his attention to Nautilie and began to sign, carefully keeping his hands out of Sans's line of sight. "THE MEDICINE WILL PUT HIM TO SLEEP." In a manner of speaking, anyway. "WHEN IT WORKS YOU CARRY HIM BACK." He paused, then added, "BACK HOME. TO THE CITY."
She opened her mouth, but before she could say anything Gaster quickly made a shushing motion. "NO, I DO NOT WANT HIM FIGHTING BACK. THE DRUG NEEDS TIME."
Nautilie huffed, but relaxed and carefully put her harpoon back in its holster before clumsily signing, "Are we saving him?"
"OF COURSE. I HAVE USE FOR HIM."
"Sickbay?"
It took Gaster a moment to realize just what she meant by the question. "YES, STRAIGHT TO SICK BAY. I HAVE A LOT OF WORK AHEAD OF ME."
"whadja gimme?" Sans's question trailed out into a long whine and he sunk into the sand.
"EXCELLENT. NAUTILIE, GATHER HIM UP AND LET US GO."
Nautilie did as she was told, being surprisingly gentle as she gathered him up. Gaster flitted worriedly back and forth, reminding her to be careful of his ribs, his tail, or his head the entire time. Then she simply floated in place, her usual deadpan expression looking particularly unimpressed as she waited for Gaster to gather up all the ingredients she'd dropped earlier. No point in wasting them!
The journey back was a long one, made even longer by how slowly they had to go to keep Sans's injuries from getting even worse. They ran into a patrol along the outskirts of the city, and of course the trio insisted on escorting them the rest of the way. Gaster grumbled to himself that they were only tagging along purely out of curiosity, but at least he was able to force one of them to carry his ingredients. Well he'd just kick them out of the sickbay once the group arrived.
It got worse once they entered the actual city, a tall, sloping cliff full of many openings (some natural, some enhanced, and some made entirely through magical means) that led into a twisting, labyrinthine complex of tunnels and chambers left over from when the mountain had been an active volcano many, many years ago. The apothecary and a small group of guards, one of whom was carrying a clearly injured monster who happened to also be a complete stranger would be sure to catch everyone's attention. As they navigated through the tunnels they were crowded by several curious monsters all shouting questions and having guesses thrown back for answers.
Gaster rushed into the sickbay. It was a rather pleasant little chamber, completely round with sand forming a flat floor and nicely sized for Gaster's needs. The walls were lined with recessed shelves carved into the rock to hold a variety of medicines Gaster had mixed himself. The ceiling was pockmarked with holes to allow in light and waterflow for the plants (all medicinal) growing along the walls. In the center of the room was a slab of elevated stone with a flat, smooth surface Gaster used as a table for whatever he was working on. In this instance it would act as an operating table.
Gaster motioned for Nautilie to lay Sans on the table, motioned the guard holding his finds to lay them out on an inset shelf deeper than the others that Gaster used as a work bench, then set about ordering everyone else to leave. There may have been some snapping of teeth involved to get his point across. Fortunately the guards took the hint and started clearing out not just the sickbay but the hallway leading to it.
"I'm going to report to Undyne now," Nautilie said after she had carefully laid out Sans on the table.
"YES, YES, YOU DO THAT." Gaster absently waved her off, then set about patching Sans back together. He grabbed a container of green magic infused salve, then stopped as he realized it would work for the bare minimum definition of the word but it wouldn't be optimal for bone. Goodness, aside from tending to a few of his own injuries he'd never needed the sticky, calcium heavy salve skeletons used. He dashed through a curtain of bespelled seaweed that hid the opening to the store room, scrambling about to snatch up his pestle and mortar, some small, clean bones, the bulb of a plant with pain numbing properties, the leaves of another plant just brimming with green magic, a container of mucus, and several other ingredients before hastily crushing them all together.
This wouldn't be near enough. Gaster glared down at the pitiful amount of salve he was halfway through making, it would do for the kinds of injuries he'd had to treat himself for and even have some left over, but there was no way it would be enough for the sheer amount Sans would need. And this all before he'd even done a full exam. He rushed back out of the store room and over to the sickbay's entrance, he hurriedly called until he caught a guard's attention, then started signing so fast he nearly tangled his phalanges together. "GUARD! I NEED BONES. SMALL. MANY OF THEM. HURRY."
"You uh… just bones?"
"THEY DO NOT HAVE TO BE CLEAN, BUT LESS FLESH IS BETTER." He paused in thought, "SHELLS WILL WORK TOO. SMALL. EMPTY. ALSO SWIM BLADDERS. BONES ARE BETTER. QUICK!"
The guard nodded and darted off. The other two stayed behind, likely to keep others out, which Gaster fully appreciated even if he didn't have the time to stop and tell them so. He went back into the room to hastily finish mixing together what he could of the salve. Once that was done he set the mortar on the table, then gathered some wraps and a small blade before beginning the long process of inspecting his patient skull to caudal fins. It was perhaps cruel to give Sans the medicine that didn't quite put him out for this, but Gaster needed the pain response. Besides, he wouldn't even remember it when he woke, it was for the best.
Gaster started with the crack over Sans's right eye. "MUST BE AN OLD INJURY," he mumbled to himself. "THIN AND ALREADY HEALED, THOUGH SCARRED." The missing fang though, that was freshly lost and likely already sea foam dancing drearily on the surface. Gaster dabbed a little salve on the stressed bone around the missing tooth, then pressed a small strip of wrap to it.
The process continued as Gaster worked his way down Sans's body, slowly and methodically testing for any gasps or whimpers from his patient. Any injury was treated with salve then an appropriately sized strip of wrap to seal it in place. Fortunately the guard returned with the requested ingredients before Gaster ran out of his batch. There was a pause as Gaster had the guard help him mix up a new batch, then he kept the guard around to help him manipulate and hold Sans's body as the inspection continued.
"NOT MUCH DAMAGE BETWEEN THE RIBS AND THE CRUSHED VERTEBRA, LIKELY HE STILL HAD ECTO DURING THE EVENT THAT CAUSED ALL THIS."
The guard sighed, but they seemed to have figured out that Gaster wasn't going to bother signing his thoughts.
The fractured vertebra was still just as much a mess as when he'd first seen it. Barely touching it caused Sans to groan and hiss in pain. Gaster almost wasn't sure it would be worth keeping, there were so many missing pieces and the raw mana holding it together looked infected. Still, if he could save it he should try. But it really would be beyond cruel to work on it without putting Sans fully out. He would finish the inspection before touching it. Gaster moved down to the mangled rays and delicately touched a cracked one, Sans didn't respond at all. His breathing didn't even change. Gaster grimaced, "I HAD FEARED AS MUCH."
He carefully inspected the rest of the remaining caudal rays, the broken ends already starting to fizz slightly. Sans still didn't respond. Gaster moved onto the hypurals, no response. The last vertebra, again no response. Gaster went back to the badly crushed vertebra and poked the next one in line, even going so far as to press a claw into the surface. Still no response. It was clear there was no saving the end of Sans's tail.
Gaster looked down at the crushed vertebra and the broken fins behind it. Sans would never swim again, even if he could miraculously save the crushed vertebra more than half of the caudal fins were already gone. Sans had already asked for death, it would be cruel to force him to live on with such an injury.
Gaster had never been merciful.
He plucked a pod from one of his medical plants, then with the free hand he parted Sans's jaws and slipped the pod inside. He forced Sans's mouth to close, causing the pod to pop and the medicine inside to absorb into Sans's ley lines. He waited for Sans's breathing to slow and grow shallow, then tested whether he would respond to a poke to the crushed vertebra. No response, excellent. Gaster turned to the guard and held up his hands, "HOLD HIM DOWN."
End notes: The first nine chapters have had their rough drafts done for a while so they should actually be coming out pretty quickly. Work's kicking my ass lately so I can't promise I'll be as fast as Skerb, but fairly quickly. Quick note: sea monsters turning to sea foam rather than dust is ripped off from Hans Christian Anderson himself, I cannot recommend the original Little Mermaid because that ending is awful but he had some really interesting world building.
Also Gaster's views do not reflect my own, he has some pretty twisted thoughts on "mercy" and what makes a person worthwhile. Don't be like him. Don't be like any of the characters in this fic, they're all pretty messed up.