The initial surprise comes in two parts—first, Papyrus's absence. He never misses a call. Ever. And the tail of the dog follows shortly after—because Sans never calls her, has never sounded like that on the phone or in person, and his words are as hollow as the empty space inside him as he tells her that Papyrus isn't coming to the training session. In fact, he won't be coming to any training sessions.
She works the truth out of him in fractured pieces, and once he admits the worst, finally, of course she's going to ask who—or what—did this to her student.
And Sans tells her.
The second surprise is when she sees it from the ledge above: The beast with a child's face and dust sticking to its cheap ballerina costume and the bedraggled, coffee-colored mane.
Undyne steps forth and it stops, hiding within the reeds. Now there is silence. It does not quiver within her presence. It waits. And when Undyne vanishes, it moves on.
And wherever it goes, Undyne will follow.
By contrast, it kills ruthlessly, recklessly. Unremorseful and hateful. She knows now that Sans was not exaggerating.
She waits, though it pains her, for an opening. She follows the trail of her prey and finds no bodies, only the dust spread over the catacombs. Breathes it in and thinks: soon.
In the natural tunnel, behind the stone pillars, she finds her opportunity. Her spears are thrown like lightning but the beast is too quick, even for her. Undyne knows it cannot evade every encounter. There will come another time. The creature will have to get tired, or cocky, or make a mistake.
She will wait.
And the next time comes on the labyrinth of conveyer belts, when the first spear finds its mark. Undyne almost crows for joy—piercing the tiny shin and the beast cries out shrilly in anger, falls to its knee—she flings more. More. She impales the little body upon the still-moving machine and imagines how its skin will catch, shred upon the unyielding metal and gears. She doesn't look away until she is certain it has stopped moving.
But half an hour later, from her perch atop the high wall with no-one left to guard, Sans calls her with the news: it came back.
On the way over to the site of new destruction, she gets another phone call. The ringtone is what catches her off guard. It's Alphys's. When Undyne answers Alphys is telling her how she's got everyone in Hotland to evacuate and the King is on board with this plan, and please Undyne, be safe. She makes her promise, even though Undyne knows in her heart that she can't afford to.
She orders Monster Kid to run while the air rushes through the gap cleaved neatly in her torso.
This thing was going to kill a child, she thinks. Her gut clenches. And Papyrus. Goddamn it. You're going to kill us all if I don't stop you.
She thinks of everyone as she speaks, in a last ditch effort to steel herself and stave for time. Saves Alphys for last in her head because she can't bring herself to think about her smiling face right now. But it comes anyway, and Undyne hates herself for allowing that emotional undercurrent.
She's fading, now. She wills herself to stay a little longer. And miraculously, the gap inside her is closing. She's stronger. Coursing with life and energy and the burning desire to achieve her purpose, at last. Reborn, she turns her newfound will into wrath, unleashed upon her challenger.
It's full of surprises, Undyne learns. Each time she hurls a spear or a volley of spikes it dodges, dodges until it makes a mistake, just as Undyne knew it would. Everyone gets tired. The weapon pierces its shoulder; it cries out like a wounded animal, grabbing the white-hot shaft lodged in its arm. Rips it out with a spray of scarlet. The flesh of its hand is singed raw, turning redder the longer it holds the weapon, but it doesn't scream, breathing heavily as it focuses on her.
Its eyes are red like the blood it holds inside its frail little body, wide with shock.
She didn't expect it to be so daring.
But the battle is a short one. The creature can't hold her spears for very long. It's forced to drop the weapon eventually, and its hand is raw and shiny. Undyne suspects it won't be holding anything for a while. The pain seems to throw it off kilter; when she pins it down and kills it, finally, it's out of distain.
As the life leaves its scarlet eyes she wonders if it will come back.
It never disappoints.
Undyne knows she should be careful. Just because she hasn't died yet doesn't mean this creature won't learn from past mistakes. She grows creative in her methods of dispatching it. Skewers it frontwards, backwards, through the mouth and chest and stomach, crushes its face beneath her boot, impaled on its hands and arms and legs like some demented puppet. There's always blood to spill.
She remembers, in particular, one time she caught it from below. She runs it through, and the point of her weapon comes out red, and the creature screams just like when she had injured it the first time, only now it's louder, choked with blood. She does not flinch. The sound rings in the silence, then fades, and the child that is neither monster nor human groans, placing its tiny hands on the shaft of her weapon and…it's trying to pull itself free.
Despite everything, Undyne sneers. "Don't. You're gonna make it worse."
It ignores her. Clawing at the energy burning right through its flesh and the smell of cooking meat assaults her nostrils. But it's a hopeless struggle; the weapon has lodged safely in its gut, melting around the wound even as the creature is panting in exertion. And Undyne is, for the first time, truly terrified at how…inconvenienced this thing seems to be.
"HEY!" Undyne shouts, more in an effort to save face, slamming the spear home, pinning the thing to the earth. It screams again, a short, harsh cry of anger, and its eyes are wide and glassy. "Don't play dead with me," Undyne snarls, "I'm not done talking, you spineless little worm." She leans down so they are face-to-face. It stares back, mouth agape. "You'll come back even if I kill you right now. Won't you?" Its eyes are bulging as Undyne presses on. "Speaking of happy returns, I've been meaning to ask you something. Do you think you could do that for everyone you've slaughtered here? Think you could bring them all back, you little freak? Or how about Papyrus, huh? Did you think I wouldn't KNOW?" She wrenches the weapon out of the stone, gouging the rock face and lifting her quarry into the air so it's suspended only by the barb in the spearhead. More blood issues forth from the child's mouth and nose as it convulses. "Just what the hell are you?" Undyne demands.
And its eyes, still red but flickering, fix on her. And it opens its mouth and more crimson oozes forth. Teeth are missing from the tiny maw. And the child slides down towards her with the pull of gravity and leaves the weapon slick, steaming with blood and entrails like the path of a snail. It reaches for her with a guttural, choked-off roar but dies halfway down.
She discards the whole weapon that time, and the trajectory carries it far into the abyss below.
Her job is never done.
Every time it comes back, it looks a little less like a child.
She cannot kill it enough times to make up for the mess it's left behind. But that doesn't mean she won't try.
It doesn't seem to care as much about pain, anymore. It catches her spears and leers at her though its palms are smoking and raw. When she kills it now it only grins.
And Undyne's getting tired. The little shit keeps coming back for more, so she gives it all she has to offer. Every death saves another life. Every minute the child wastes here is for their benefit.
The air smells less like the damp murk of the cave and more like salt and iron. The bodies pile up. Dead weight.
This can't go on forever.
And even a soldier as proficient and ruthless as Undyne is going to screw up at some point.
Death is unexpected.
The child smiles wide, and its teeth are white smeared with red and grey, mouth twisting into an animal snarl. Undyne can barely stand when it catches her spear, looks up and smiles like it's doing her a favor or something. Cruel and wicked and ecstatic. It's enjoying this. She is the first challenge this thing has had in a long time and Undyne snaps. She tries to rip the child away but she's weak. And hatred glows in its scarlet eyes as it twists the great weapon with far more strength than she thought possible of such a frail body, and impales Undyne on her own weapon.
The heroine does not open up like a sandbag. She has no blood or innards to spill. Her body congeals around the shaft of her own weapon, steaming with the electric heat. Her mouth is open but no sound comes out.
The beast in child's skin twists the spear hard and she blacks out for a second, it's so painful. And Alphys is screaming so loudly the speakers blare with feedback but Undyne cannot hear her.
The beast lets her sink on her own weapon. She tries to groan but can't. Falls to her knees and it isn't smiling anymore. It looks down on her like she's nothing but a shiny scuff mark on its shoe.
"If you think…" Undyne gasps, "that I'm giving up—"
She never gets to finish her sentence.