Prologue:

The ominous, echoing clanking of armoured feet heralded the approach of the end for the settlers of Darkwater Pass. Alex crouched behind the hastily erected barricade of overturned bookshelves, tables and bits of rubbish clutching her rusty old Mossberg. She knew perfectly well that if they decided to attack she and her family wouldn't stand a chance. But there was a chance. As long as she was leading the group and not that thing they called captain. Alex didn't hold out much hope though. Her idiot brother and his friends had wiped out one of their scout patrols, sniping at them from the bell tower of the old church and laughing as though it were mere sport. Now they were in this mess.
Damned fool she thought. If he weren't such a good shot I'd kill him myself. These days everyone and their mother (if they're mother were still alive) knew that you didn't piss of The Kingdom. Not unless you had a serious death wish. So what does what does dumb, drunk-ass Eric do? Kill three royal scouts and bring the entire royal guard down on their tiny, barely surviving, little settlement!

The Kingdom was either your best friend or your worst enemy. If it weren't for her stubborn old codger of a grandfather and his damned pride she would have left Darkwater and moved to New Ebott years ago. But he had insisted that they were perfectly capable of looking out for themselves in this hell blasted wasteland they called home. He had refused help from a "a bunch of damned mutants". Alex thought he was stupid but being mayor of Darkwater, and the only man in town to own a functional assault rifle there hadn't been any arguing with him. At least The Kingdom had respected that and left them alone. Now they were probably about to die. The clanking of armour had grown louder as Alex had contemplated her impending demise. Alex peeked over the barricade and saw them coming. The great black armoured forms of the royal guard's elite warriors seemed to absorb the light from Darkwater's spotlights as they emerged from the mist and darkness of the night, as though the darkness itself had taken form and come for them. Which considering it was The Kingdom, was entirely possible.

Fifty feet from the barricade the two colossal armoured figures stopped, rifles in hand, they seemed to bob and sway where they stood as though so eager for a fight they couldn't manage to stand still. After a moment's silence the two figures parted and a third emerged, a fourth following a little further behind. Alex felt the smallest of hopes flutter in her chest. The figure who stopped before them at the barricade wore no armour, save for a bullet proof vest over a blue hoodie and a striped t-shirt. They carried no weapons. No holster hung from the belt that fastened with a Delta Rune shaped buckle, the symbol of The Kingdom, around a pair of blue jeans that looked as though they had just been cleaned. And their boots, though coated with the dust of travel, looked almost new. It was her 'The Kingdom's Angel of Mercy', 'the girl who walks with monsters', Frisk.

"What do you want Kingdom?" Demanded the age cracked voice of her grandfather, no longer cowering behind the barrier but standing straight, facing the invaders, his chin stuck out defiantly. The young woman took another step forward, smiling kindly but didn't speak. "Hey now!" her grandfather croaked, "Don't you come no closer! I don't have anything to say to a race traitor like you." Alex winced, now really wasn't the time for her grandfather to voice his true feelings, they were supposed to be trying to negotiate for their lives, not provoking them. The angel staggered back a half step as though grandfather's words were a physical blow. The look on her gentle face was one of shock and hurt and Alex thought she might cry. But after a moment she seemed to compose herself. She straightened and fixed Alex's grandfather with eyes that barely seemed to be open but she knew saw everything. She wasn't smiling anymore, but neither did she look angry. Alex was still hopeful.

The angel took another step forward and this time Alex's grandfather raised his weapon. "Not one more step!" he barked, sounding quite forceful for a man of seventy years. The response was instant. The two colossal figures took a unified step forward, raising their own weapons. Alex swore the ground shook. There was a sound somewhere between a growl and a hiss and the fourth figure advanced rapidly to stand beside the one called Frisk who suddenly bore a look of panic and threw up her hands as though to both apologise to Alex's grandfather and to order her guards to stay back. The fourth figure, midway through unsheathing the two handed sword across her back froze mid step but the visor of its helmet remained fixed on her grandfather. There was a white glint there and Alex was sure that it was glaring at him with intent to kill. Eventually Undyne, for it could be no other than the captain of the royal guard with her ornate flanged armour and gigantic sword that no regular man could ever hope to wield, nodded once and stood at ease behind Frisk, her helmet now tucked under her arm. Her mutated features were surreal yet, oddly, not unpleasant to look at Alex decided. Her blue scales and red fins were completely alien, as were her fangs and yellow slitted eye, but her long red hair pulled up in a tight ponytail and the shape of her head was more human than fishlike. The scars on her battle hardened face, and the eyepatch over her left eye said that she was a killer. But the way she watched Frisk with the protective air of a big sister told Alex that, at least underneath they weren't that different.
Still, Alex thought, she must have been exposed to an ungodly amount of radiation to mutate her that badly.

She was pulled from her contemplation when, this time, Frisk looked to Alex. And she suddenly felt nervous. Did she think that Alex was more level headed? She was right of course but she didn't know if her grandfather would take it as an insult.
"I'm Alex" she blurted out, and then felt like a fool but Frisk smiled slightly and gestured for her to continue. "We're very sorry about what my-" she stopped abruptly, not wishing to reveal that her brother had been one of those who had killed the scouts, "-that our people killed some of yours. We are only a small settlement and our watchmen sometimes get jumpy. As you can imagine, with a settlement our size even a single raider can be a problem."
"And you Kingdom Bastard's can-"
"GRANDPA ENOUGH!" Alex yelled, and Frisk actually jumped. "You aren't helping me save our lives here!" She glared at him for a moment. When he grunted and fell silent she tried to compose herself and smile back at Frisk who had by now moved so close they were no more than an arm's length apart. "Sorry about that" she said. "Anyway we are willing to offer The Kingdom recompense for our mistake, just tell us what you want." For a moment Frisk's smile seemed to flicker, flashing between gentle and psychotic, but Alex was sure it was just her own nerves making her see things.

When Frisk spoke her voice was gentle, musical and barely audible even in the silence of the night.
"We want you to join us" she whispered.
"What?" Alex asked, completely stunned. She had expected them to demand monetary reparations, maybe goods, servitude or possibly even the lives of those responsible.
"You said your life out here is hard. Life in The Kingdom would be a paradise, food, shelter, and most importantly safety. Don't you want that for you and your people?"
"W-well yes but I just thought-" she cut herself off again before she could say anything stupid. This was better than she could have hoped for! "I mean, yes of course I'd be delighted to accept but-"
"Hey little miss whispers. I'M the one you should be talking to!" Alex flinched as her grandfather's irate voice cut the night again. Frisk's face seemed to harden as she turned to him. "Whatever you want you talk to me. I'm the mayor of Darkwater."
"Of course" Frisk said, but the kindness in her tone was gone.
"Now we'll give ya compensation for the damage ma idiot grandson caused you. Then you freaks can leave us alone."

"Grandpa" Alex said through gritted teeth. "They want Darkwater to become a part of The Kingdom. I think that-"
"No way!" he growled immediately. Alex shut her eyes in horror. She knew that had been coming.
"Grandpa please. This is our chance to be safe, to not have to fight roaches over every scrap of food, or feral dogs for every kill."
"If I've told you once I've told you a thousand times we don't need protection from no damn stinking mutants!" Frisk didn't look gentle now, she looked angry. Her voice didn't get any louder but there was an edge to it.
"Then we need something else."
"That's more like it" Alex's grandfather said, looking smug, clearly believing that his senile ranting had won him the argument.
"Give the guilty men to us and we will leave you alone." Alex blanched and her grandfather's smug smile disappeared in an instant.
"Now hold on there. Compensation doesn't equal letting you kill my townspeople!"
"We will not kill them but they must pay for their crimes. The Kingdom has a fair and just legal system. They will stand trial and if they're found guilty they will go to jail, as any criminal should."
"A Kingdom jail where the guards probably eat prisoners or something!" Grandfather roared in outrage.
"HEY!" Undyne barked, speaking for the first time, "that only happened ONCE! And I fired the guy who did it anyway!" Frisk turned to glare at the guard captain. "What?"

"You see Alex? They're all insane. They're freaks and she's a traitor to the human race!" He was backing away now. His eyes wild and Alex felt dread pool in her stomach. "I'll not feed my people to these mutant bastards." Alex's eyes widened with horror as her grandfather raised an open palm in the air.
"Grandpa don't!" she cried desperately, but it was too late. Her grandfather's open palm closed into a fist. From the church tower a shot rang out and Alex knew that Frisk was about to die. There was a ping and Alex opened her eyes to see Undyne crouched in front of Frisk, sword drawn a small scratch on the gleaming face of the blade where the bullet had struck it and bounced off.
"Bad move old man" Undyne hissed, all of her fangs bared in a menacing snarl. It vanished to be replaced with momentary confusion as Frisk's hand appeared on her shoulder. She stepped around her armoured guardian who smiled again when Frisk handed her the helmet she had dropped.
"Thanks punk. You're still pretty useful as a squire, even if you've lost your edge for negotiation." Frisk grinned at what was obviously a just a jibe, but Alex could still feel dread as she leaned over to whisper something in Undyne's ear (fin?) that Alex couldn't make out even standing next to them. "Asriel will be happy. It looks like you just lost a bet" Undyne muttered in return before rising to her feet.

Frisk turned to Alex and took her hand in both of hers.
"Run, take no weapons and don't fight. Tell those who will listen." Alex didn't understand, until Undyne donned her helmet and straightened fully. She sheathed her massive sword and raised a finger to point at Alex's grandfather.
"Destroy them" she ordered, "take only the souls of those who fight you and bring the rest to us!" Alex watched in horror as the two colossal figures charged forward, followed by a dozen more that had hidden in the darkness. Alex stood petrified as her grandfather opened fire on Undyne. She turned to him unimpressed and impaled him with a glowing spear that manifested itself from nowhere. Alex ran.