I wrote this story mostly for loverofpiggies because of her pretty baller Error Sans meets Underswap comic and I guessed it inspired me or something (this story is following the tenth installment btw)
So yeah, first Undertale story (of many to come! I'm lazily working on a sanster chapter fic all the while attempting to finish a Gravity Falls fic I've been working on for far too long). Loverofpiggies, if you're reading this, your comics are awesome and I hope you enjoy :D
The last he ever saw of Papyrus was his agonised expression. His hand was outstretched, reaching for him but too far away to hope to even graze his own hand, and he was shouting. Sans had never quite heard his brother sound so torn and desperate in all his life. It broke his heart and inspired worry that hadn't been present until a few moments ago. And where he found the sight and sound of Papyrus to be utterly heart wrenching, Error found it maddeningly hilarious. He cackled so terribly it had Sans whimpering and tensing. This bad monster – this glitch had just stolen him and was steadily closing the doorway temporarily binding their realms. Though he might have once had a chance to appeal to Error's glimmer of good nature, that chance had all but disappeared now.
"Please! Wait! Take me instead!" Papyrus begged through the last holes of the closing portal.
"I don't want you!" Error snarled and then the portal closed.
Sans was stuck in a white void. He looked around and found there was absolutely nothing aside from himself and Error. The latter stared at him curiously and it was unnerving. Sans refused to meet his gaze and he could tell the other found it intruding. He spotted a distorted smirk on Error from the corner of his eye as he began to stroll towards him. Sans panicked. What was he going to do? Was he going to hurt him? He hadn't been above fighting Papyrus, though he wouldn't have hurt his brother as much as killed him what with his 1hp. Sans had 20 hp thus meaning Error could torture him if he wanted. Was that what was going to happen? He had been a fool to believe kind words and good intentions could solve anything with this foe.
"S-stay away!" Sans shouted and took a few steps back, nearly tripping over his feet. Strangely, Error stopped and cocked his head to the left, "Don't come near me!"
"Or what?" Error sneered, "You'll kill me? You've never been able to kill a thing in your life."
Sans flinched and lowered his head. He stared into the white abyss below and willed his trembling to still. What was he supposed to do now? He couldn't bluff because Error knew he couldn't – wouldn't kill. He gritted his teeth together and shut his eyes. Sans was scared. He wanted to see Papyrus. He wanted to bury his face in his brother's chest and cry while he was told that every was okay; that it had all just been a bad dream. When he opened his eyes again, a part of him truly thought he would see Papyrus in the last corridor. Instead, he saw the endless white and, in the midst of it, Error. Sans averted his gaze instantly but he noticed how the latter bore an indescribably expression. He couldn't tell what was going on through the glitch's mind and that much was terrifying. Then again, even if he could anticipate what would come next, he doubted he would find it the least bit comforting.
But after a long silence, Sans reckoned anticipating his inevitable doom would be better than mindlessly waiting for it. It was beginning to get unbearably unnerving to just wait under the maddening gaze of Error who was just staring at him. It almost felt like he was waiting. For what? Sans couldn't fathom for the life of him. Perhaps Error wanted him to beg for death. If that was the case, he would wait eternally because Sans loved life and feared death too much to ever beg for it. And yet, a part of him reckoned that that wasn't it; that Error wanted something else. He wasn't staring at him with a cruel expression, thus Sans thought it was safe to assume his life wasn't in peril.
Yet.
"What…" Sans started in a shaky, small voice. "What are you…looking at?"
"That's a dumb question." Error snorted.
Sans silently conceded it was.
"Then…why…" he tried instead, "Why are you staring at me…?"
"Because there's nothing else to look at." Error answered, grinning widely. He seemed to find amusement in Sans's dull questions.
Sans forced himself to remain both calm and patient. "What do you want…?" He asked.
"You." Error replied simply and Sans didn't know what to make of that.
What did that mean? Did…did Error want to do lewd things with him? Aside from not wanting to, Sans was fairly certain it was impossible what with being a skeleton. Papyrus had also told him the Gods would cry and destroy the universe if he ever attempted anything of the like. In the end, Sans concluded that that wasn't it either. If it was, then Error had the power to make him submit whether he wanted to or not. Instead, the glitch was just watching and waiting from the point at which he had stopped once Sans had told him to stay away. The notice sparked confusion. Why had Error stopped anyways? Sans had no power against him let alone in his realm. Couldn't he just take whatever he wanted?
"What…what do you want from me…?" Sans asked and he wasn't sure he wanted to know the answer, but waiting was getting insufferable.
"Nothing." Error shrugged, "Just you."
Again, Sans's mind wandered to doing lewd things. The characters from Alphys's anime tended to say similar things when they were romantically interested in others. Was that what Error was doing? Was he confessing and consequently expecting Sans to kiss him despite not having lips? That would be rather surprising in itself seeing as they had met only moments ago and Sans didn't think emotions could develop so fast. Then again, he was pretty great so perhaps it was plausible. But he was a skeleton with standards and he wasn't about to start dating anyone just because they had a crush on him. He had learned the hard way that misleading someone with such sincere feelings only lead to more hurt and pain.
"Uh…listen…" Sans started slowly. Rejecting someone was always a touchy situation but with Error it was more so because he had the power to decimate him with a brief flick of the wrist. "Your feelings are…flattering, but I…I don't feel the same way."
Error said nothing but his confused expression relayed his thoughts.
"I don't feel the same way so, uh…" Sans continued nervously, "If you…if you could let me go home…that would be great…"
Then Error began to laugh. It was a loud guffaw that seemed to fill every inch of the white void. It was unsettling to say the least and put Sans on edge. He just wanted to go home, to be with Papyrus again. To pretend all this had been nothing more than a bad dream.
"I don't want you like that." He snickered and took a step forward. When Sans took a step back, he stopped again. "Even if I did, I could see I have no chances. You love that brother of yours."
"What? N-no I don't!" Sans squeaked and felt his cheeks heat up with a blush, "I-I mean, I love him, but like a brother. Nothing more."
"Well, he obviously doesn't feel the same way." Error smirked and sat down.
"Why are you sitting down?" Sans asked and he wasn't sure whether it was cause for worry.
"You don't want me to come closer." Error responded, "So I'm sitting. It'll keep me from feeling tempted to move even if you don't want me to."
"Why…?" And the more he talked with him, the less Sans understood him.
"Because you don't want me to." Error repeated.
"But why does what I want matter?" Sans asked, losing a bit of patience, "Why don't you just take what you want and let me go home?"
"What I want isn't something you take, or so I've been told." Error explained, "It's something you earn."
"But…what is it?"
"Your friendship."
So I'm not actually sure how much hp (canon) Papyrus has but I reckon, because he wouldn't kill another monster, he would have the standard 20 hp. By extension, Underswap Sans would have the same amount of hp. Does that make sense?
But yeah, based off the comics made by loverofpiggies, I think ErrorSans kidnapped the blueberry because he was lonely and Underswap Sans is just such an adorable blueberry (Lord knows I'd kidnap him just to have something to snuggle).
So this is my theory, I guess, as to why Error spirited Underswap Sans away.