You hold your smile like someone's hand while you watch them all take in the sunset. It's beautiful, even from back here beneath the last canopy of the Underground's caverns, fiery red-orange and expansive. The sky goes on forever, and it's so far away. You had forgotten what that felt like.
Frisk holds Mother's hand, so small within her paws it almost vanishes, and all along the edge of the cliff stands all their friends you have come to care about as you have followed them, admiring the sun you've never seen.
Papyrus is probably the one you wish you could meet for real the most. He seems so kind and empathetic, and Asriel said he had been friends with Papyrus in his own runs. You think you could have been friends if fate had allowed.
Alphys is a strange woman you pity more than like. Frisk trusts her, but you don't, not entirely, but you don't think she's a bad person. Just a foolish one who bowed to cowardice- and hopefully, she won't again. She's alive. She had time to grow and change and be someone new.
Undyne is basically your hero. She's amazing… breathtaking, awe inspiring, passionate and dramatic with a flare for monologues and action, all the things you like, ready to die for her beliefs and loyal beyond words to those who had been kind to her! Undyne is the hero the Underground always deserved.
You don't particularly like Sans. Something about him gives you a bad vibe. Perhaps he's not allowed horrible things to happen like Alphys had, but something about him tells you he would if fate had put him in a position he could have stopped them. Seeing his darker inner thoughts while you were calling to the lost souls of Frisk's friends made you loosen up some on how you felt, at least. But he isn't your friend. You don't have to trust him.
Mother, too… you get the feeling she's long since moved on from you. Still grieving for Asriel, but… she has buried you, and moved on. She waters the flowers on your grave, but she weeps for Asriel. You suppose that fair. You did take him from her.
And Frisk… You barely know Frisk. You have followed them their entire time here in the Underground, but… you have done most of the talking. They're kind, empathetic, intelligent, but… what do they like? Where did they come from? Why did they climb Mt. Ebott? You don't know. They don't speak to you. You're not completely sure they know you're there.
The sun is setting, and Frisk's friends begin to disperse, leaving only them and Mother. Watching them hold hands in front of the sunset, whatever broken pieces remain of your hurt ache to go join them, a sharp pain in a thousand fractured splinters between your ribs.
Frisk and Mother turn, walking along the path, and just before they vanish from sight, Frisk turns back. You aren't sure if they're looking at the exit to the Underground or directly at you, but- a moment later, they're gone.
Both of them.
Forever.
The sun dips ever lower on the horizon, bringing forth red-magenta-violets that ripple through the sky and give way to peppered stars. Everyone will leave, soon, to enjoy the beauty and life on the surface. It's what you always wanted, what you were willing to die and kill for, what you have been waiting for over a hundred years.
The ground beneath your feet now is clean and smooth but soon it will be trampled by hoards of people leaving, but not you. Your feet will leave no footprints and you will not join them. Even as the last of the freed trickle out you will be here, standing at the exit, holding your smile like a hand. You can feel it, within the very fiber of your being: you cannot take a step further. You cannot leave this place.
Soon, everyone will leave for the surface, just like you always wanted. You will be the last monster in the Underground, forever.