“G’night, mama goat.”
Undyne grinned toothily down at Toriel and Frisk, the motherly goat monster with book in hand, the kid tucked contentedly into bed. Papyrus, Sans, Asgore, and Alphys had all already said their goodnights to their newfound little sibling and retreated to their rooms, but Undyne had been called upon to provide some choice voices for Frisk’s bedtime story, and so had lingered.
“Good night, Undyne.” Toriel smiled warmly. Frisk tugged at Undyne’s hand one last time; she squeezed it with a smile and a “G’night, punk,” and then turned, making her way out of the peaceful room.
She could already hear loud snoring coming from the skeleton brothers’ rooms as she went down the hallway. Since reaching the surface, their little family had come together to look after each other, after Toriel had officially adopted little Frisk and none of the others much cared to part ways in such a strange new world. Now they shared a modest-sized home in the first new monster colony on the surface. The humans had, of course, been considerably worried and nigh-on hostile upon seeing the monsters for the first time, but Frisk had been a great help in settling peace, and soon they were co-existing, making friends...everything the monsters had dreamed of for so long.
And even more than that, Undyne had someone to share her dream with. She pushed open the door to her bedroom, her normally cockily playful expression softening at seeing the little dinosaur monster hunched over her desk, tinkering with something. She knew Alphys missed Mettaton greatly, for the robot had elected to stay in the Underground to entertain those monsters who remained; maybe she was making a new robot. Not another Mettaton, of course; nobody could ever quite match that personality.
Alphys lifted her head, giving a smile at seeing Undyne. “Hey.” she said, softer.
Undyne bent to give her girlfriend a kiss on the end of her rather long snout. “Whatcha workin’ on, squirt?”
“Oh - “ Alphys glanced at her workbench. “J-just - I don’t really know yet. Just something to keep my claws busy.”
Undyne scooped her comparatively tiny girlfriend into her strong arms, making Alphys squeak in surprise, but quickly relaxing into her embrace. Undyne grinned and collapsed back onto their bed with Alphys coming to rest on her chest, her body and head lying out on her torso and her tail wiggling between her bent legs.
“Well, now you can keep all of you busy with cuddling with me.” Undyne said, making Alphys giggle and blush. Undyne pecked her adorable girlfriend on the nose for that.
“Y’know, Undyne…You’re a big softie.” Alphys mumbled, lying her head sideways on Undyne’s chest. She could hear and feel the fish lady’s laugh.
“Y’ got that right, squirt. Gotta take off my armor sometimes.” Undyne rubbed her hand up and down Alphys’s back, massaging the tense muscles she found there. Alphys hummed contentedly, almost comparable to a cat’s purring. Undyne let her eyes drift shut, feeling a warm sense of contentment and love. Alphys’s purring soon lulled her to sleep with the little reptilian monster’s head nestled on her shoulder.
Undyne woke to a scream. A feminine scream of loss and fear - suddenly cut short - it was Toriel’s. Her eyes open wide, she clutched Alphys to her chest, sitting bolt upright and an energy spear immediately forming in her hand.
She heard Sans’s gaster blasters coming to life in the room next to theirs, and heard the smaller skeleton run out into the hallway, but he didn’t get far before - whoosh. Undyne’s breath seized in her chest. She got up, slinging Alphys to cling to her back, and shoved their dresser drawers in front of the door, trying desperately to stamp down the fear leaving her muscles trembling. Whatever this was had had enough surprise and strength to kill Sans -
She ran to the window, swinging her fist at it but was met by something like an invisible wall of some strange energy, repelling her hand. Alphys gave a terrified sound, her arms tightening around Undyne.
Suddenly, the door burst open.
Undyne whirled. It was Frisk, a long, gleaming knife in their hand, and dust covering their head and shoulders. Their eyes glowed red, as if possessed by some malevolent demon.
She dropped Alphys to the ground, shoving her underneath their bed and standing in front of her. “Don’t come any closer!”
Frisk paid her no heed.
Undyne threw her spear, but the small child dodged it nimbly, and before Undyne could even react or summon another spear, she felt a white-hot, agonizing pain across her chest.
She slumped to one side, her vision wavering. She could feel herself dying - but it wasn’t so much a sudden dust, as an agonizingly slow melting. She heard Alphys scream in terror; and she opened her eyes just long enough to see Frisk overpowering her wriggling just long enough to stab their knife deeply into her back. Alphys’s eyes bugged out, finding Undyne’s with an expression of pure terror before she burst into dust and disappeared.
Undyne gave an unintelligible cry of grief. Something inside her told her she could come back from this - some determination - but… Alphys was gone.
Frisk stalked towards the dying fish woman. Even in her state, Undyne realized this wasn’t Frisk - their mannerisms were all wrong -
“Hello, Undyne the Undying.” Frisk’s voice was low, gravelly, but almost feminine. Very un-Frisk.
Undyne gaped. She was almost all gone now. “W-who are you?”
Frisk stood over her. “I am Chara.”