After turning a few corners, the cave tunnel widens and you see another large stone door, like the one the other you from the stable timeline let you through. It looks unbreakable, and when you try, you can't get enough of a sense of the other side to teleport past it. It might be a magical barrier too. You wonder what you're going to do now, but as you approach it and the bones of your phalanges touch the smooth surface, you hear heavy bolt locks on each side withdraw into the surrounding wall. The door rumbles and lifts up on its own, letting you through without issues. Maybe somehow the door recognizes you? Maybe the other you who opened the first door somehow told all the other doors to let you through.
There's a short hallway after that, and another door, which also unlocks itself and lets you through. This pattern keeps repeating several times.
"come on, guys, how many doors did you have to set up to block the path here?" you grumble.
The pounding sound gets louder and louder, until finally you're sure that the one doing it is on the other side of the next door. All of the doors you've come through have closed tight behind you.
This final door is very different than the others. Instead of opening right away, it has a turn wheel to open it, like what you'd see on a heavy vault door. On its stone frame, there are several large reinforcing strips of steel bolted on it. The huge vault lock and the additional steel strips look newer than the rest of the door, as if the door was damaged, and then parts of it were replaced and reinforced.
Well, you're here, so no turning back now. You take the wheel and begin the process of manually releasing the heavy lock. The wheel makes a clicking sound as it turns. As soon as you start to turn it, the pounding behind the door stops.
It takes awhile and several complete turns of the wheel, but finally the lock is released and you hear the bolts being drawn back from both sides of the door. The door moves up into the ceiling like the others.
As the door moves up, you see another you on the other side, about thirty steps away, in the middle of a large cave. He's watching the door intently as it opens.
His jacket is a darker blue than yours, with white trim that looks fuzzier and finer than yours. The strangest thing about him is his magic and eye lights. They're both black, which would make them hard to see normally, except that they're outlined by white around the edges.
The other Sans has a relaxed stance, his hands in his jacket pockets, but his magic attacks are at the ready, with several large bones and a Gaster Blaster on each side pointed directly at you.
You've been in too many battles to simply walk into an attack. When you see his attacks are ready, you quickly dodge behind the doorframe.
There's a stalemate for a few seconds.
"Keep your magic down and I won't attack you," he says, his voice rumbling across the cave floor between you. "If you don't want to come in, then leave. There's nothing to see here but me."
You peak around the door. His attacks are still up and ready, but he's pointed them away from you. That's not great, but it's better.
There's torches with blue and white magical fire lining the walls of the cave, so at least you can see the area clearly. You notice that there's several rocks of various sizes concentrated mostly near the edges of the cave that you can use for cover if you need to.
You nervously walk through the doorway. Behind you, the door closes again and locks itself.
You shift into battle perception as you walk towards him, allowing you to see his soul and magic more clearly. His soul is also black outlined by white. His soul is far more intense and powerful than yours.
He's grinning, but it's the same kind of skeletal grin you show Frisk before a fight. He's looking you over, not moving yet, but ready to attack you if you give him the slightest provocation.
But when you look closer at his expression, you see a sadness in his eye lights.
When you see that, you make the decision to shift out of battle mode and return to normal vision, relaxing your stance, and putting your hands in your pockets like him.
He looks you over with interest, and then breaks the stand-off with a question.
"So. What are you doing here?" he asks bluntly, in a font that's anything but casual. He sounds defensive and unfriendly, as though he expects you to ask him for something he won't like.
"eh, i was in the neighborhood, picking up groceries...you wouldn't believe the kinda bargains they have around here. it's real nice place, glad i moved in. i forgot to buy some tomato sauce, though, so i wondered if could borrow some," you drawl, as if you're just showing up at his house from next door.
His jaw tenses slightly, although the rest of his expression doesn't change.
"it was really tough trying to get to your front door though. you know, if you want people to be able to visit you, maybe you shouldn't put a huge vault lock on it. doesn't help if you put it so deep in the earth that no one can find it, either."
He shrugs. "That's what you do with crazy people. You lock them away. Don't know if you've noticed yet, but I'm not here by choice."
So that's it. The other Sanses think this Sans is crazy and dangerous, so they locked him away down here. The destruction and cave-ins you saw on the way here says they're right. Maybe he broke out before, and caused havoc. That would explain why you had to go through so many locked doors to get here.
But you still have a bad feeling. Maybe there's more to this.
"you don't look crazy, though," you offer. "why do you think you are?" You're not really sure if he is or not, but you're trying to be diplomatic and get more information out of him that way.
The sadness in his eye lights grows more intense, and he relaxes the magical weapons he has floating in the air a little.
"That's what they told me. When I talk to other me's, they act like I'm crazy and dangerous. I get mad, we fight, and then they run away. Eventually a lot of them decided to gang up and lock me in here. That way they never have to see me, or talk to me. Sometimes they have an expression like even seeing me is painful. They act like I'm a...I'm a..." He struggles for the right word.
"a monster?" you suggest.
He nods. "They look at me sometimes like some of the humans look at us monsters on the surface. Like we're things that shouldn't exist, especially if we're skeletons."
Why, you wonder. He seems normal enough, even if his magic is an unusual color. Just because he's not a common Sans, that's no reason to lock the guy up.
"why don't you tell me what gets said before the fights start? maybe that'll help," you say.
He hesitates and you wait for his answer.
"I don't think you want to know."
You clench your teeth. "i heard that from the other sanses too. i dunno what the deal is, but something's not right here. i want to find out why."
He hesitates, struggling with what to do. There's a desperate loneliness in his expression, like he's afraid you'll treat him like the others did, and yet he really wants to explain and be heard by someone who understands.
He finally answers you very slowly, his bass voice deep and heavy with emotion.
"I tell them I'm tired of dying for them. It needs to stop. I can't take it anymore. I tell them if they don't stop making me do it, I'm going to start making them remember."
Like lightning, his words shoot through your mind. Your bones lose their strength and you fall to your hands and knees. Your breath quickens like you're having a panic attack right there.
"No! no no no no!" you scream inside.
"See? It's already happening to you!" he says despairingly. "Are you going to fight me too?"
"you're nuts. you're crazy," you hear yourself say, like it's not even you anymore. "dying for us? what does that even mean? i deal with it when it happens. it's not that bad. i've even learned to deal with the pain." You're just trying to convince yourself now, your voice low and panicked.
"No, you don't," he answers. "You deal with it for a few seconds after it happens. Then you make me take over."
"i don't believe you," you tell him, your voice like stone. "they were right. you're just insane. that's why you're down here, right? you tried to attack the others as they ran away. i saw the evidence in the tunnels coming here."
"I won't listen to this anymore," he answers. "If you're just like the others, if you're going to keep me locked up in here, then the least you can do is LEAVE! You have five seconds to get out of here!"
His words are lost on you. He's a threat, and all you care about now is taking him down. You feel like nothing more than a raging beast now, acting out of a survival instinct to eliminate the threat.
You attack him with all your strength, trying to hit him from all sides with all the bones you can create with your magic. But to your surprise, he uses gravity magic to snatch them all from the air and slam them against the wall to your right with a skillful flick of his left wrist.
They pound heavily against the rocky cave wall, making a huge mess as several bones longer than two meters each break apart and shatter at an extremely high speed, from the redirected force that you put into them. A few of the magic lanterns on the wall are destroyed, their fire stuttering in a way that makes their light flash eerily before they finally wink out.
Then he grabs your soul with gravity magic and lifts you in mid-air, confining your magic with his own so you can't attack him.
In that moment, you feel how powerful and strong he is. You get the sense he could crush you in an instant if he really wanted to. He clenches his fist for a moment after he grabs you, his jaws clenched in anger, and you gasp in pain, until he seems to stop himself and loosen his grip so he doesn't hurt you too much.
But his mercy is lost on you as you use all your inner strength to wrench your soul from his grasp. He's surprised but manages to recover and dodge the bones you throw at him to distract him as you teleport a short distance away, behind him and out of his visual range. You launch another attack, and he dodges again, looking pensive, his hands in his jacket pockets.
You teleport to random locations around him, preventing him from getting a target lock on you as you launch spiked bones from all sides of the cave.
He teleports and dodges the bone missiles with extreme skill, far more than you're capable of. He never launches an attack at you. He just keeps dodging, and you wonder how long he can keep this up.
You wonder why he doesn't try to take control of the bones you send at him and redirect them, like he did before. A small inner voice whispers that the reason is probably that you're both moving so fast that he's afraid he'll accidentally hit you with them, either directly or with the shrapnel if the bones hit the walls at a high speed and shatter. You ignore that. He's a threat. That's all that matters.
When a bone from one of your attacks is about to hit his skull, his eye lights focus on it as your soul heart clenches with regret. Some part deep inside of you feels that this is very wrong, and is relieved when he successfully dodges it.
Your inner conflict makes you lose your focus for an instant, and in that short space of time he manages to grab your soul again with gravity magic.
You struggle furiously, but this time he is not gentle with you, and holds you strongly. You pant shallowly, your mind a haze. He's definitely hurting your soul a little with such a tight hold, but you also feel strangely relieved that there's no give in his hold when you try to push against it.
He keeps his left hand raised to maintain his hold on you, his brows lowered and his eye lights dark and intense like two swirling black holes, but still edged in a white light. He tries to talk to you again, his voice rough with emotion, and deep and loud enough that it rumbles and echoes through the cave.
"Do you remember that look that Frisk has when they've killed everyone? That dead look, like all they want to do is kill you too, and that's all that matters to them? That's the same look you have now. You don't care about anything other than destroying me. Do you remember how it feels to be on the other end of that? How we've wondered why the demon controlling them has a soul so dead that they can't see that the people they're hurting have feelings and the ability to feel pain? Do you think I won't feel anything if you hit me with an attack like the one you just sent at my skull?"
A slight trickle of reason makes its way back into your mind at his words. Yes, you do remember that, and it feels awful. It feels beyond horrible, actually. It's never just the physical attacks that hurt so much when Chara attacks you, using Frisk. It's the raw malice in her strikes, the way she hates you so much and wants to destroy you. Malice hurts monsters more than anything else, as you know, maybe better than anyone.
It's so hard to get your soul to stop its mindless rage. You start to see that you're the one in the wrong here, but for some reason it's extremely difficult to calm your mind and convince it to stop attacking. Your mind seems to be irrationally convinced that this Sans is somehow an existential threat, and if he is allowed to live, you're going to suffer some kind of horrible fate and maybe even die.
But you don't want to be like Chara. You don't want to hurt anyone. You finally get your mind to agree with you on that much, so that you manage to stand down your magic. This is the equivalent of an offering of peace during a fight between monsters.
As he sees you do this, the other Sans lowers you back down to the cave floor and releases his hold on you.
Even though you recognize his merciful actions by letting you go, you can't bear being near him anymore. You turn and run for the exit, the door you came in. It lets you through, as though it detects your desire to leave and recognizes that you're not the prisoner it's meant to contain.
You go through the doorway and the reinforced stone door closes again behind you.
You lean back against the door in the small hallway beyond, outside of what you now know is essentially the black Sans's prison cell, panting heavily.