Notes: So, I watched 3x20 a few hours ago and I saw briarsrosie's tumblr post on "when your flashfrost villain au comes to life" and I was so inspired by that that I sat down to write something. I also read someone's post speculating on how Savitar gave KF her clothes, so I incorporated that too. I'm really sorry but I can't remember the source. I'd love it if you let me know so I can link it here.

Title is the song "Everybody Wants to Rule the World" by Lorde (originally by Tears for Fears). I listened to Lorde on loop while writing this. It's an amazing cover and it does wonders for setting the mood here.

i.

Welcome to your life
There's no turning back
Even while we sleep
We will find you
Acting on your best behavior
Turn your back on Mother Nature
Everybody wants to rule the world

He's watching her.

She knows this even with her back turned to him, even with his suit locked in place, even with the mask shuttered over his eyes. He is always watching her, and he has never tried to hide it.

"So this is your little hideout," she drawls, running fingers over the grimy metal, leaving a trail of frost in their wake. "A bit of a dump, but I suppose it will do."

"I've brought you clothes," he says. He throws a sack onto the floor. "Wear them."

She narrows her eyes at him. "A costume? Really?"

"Unless you want to stay in Caitlin Snow's clothes?" he returns.

She looks down on what she's wearing. A tight pencil skirt, a hideous top, and a torn lab coat. She frowns. "No. This is appalling."

"Then wear these."

"You could have at least handed them to me like a proper gentleman," she mutters. She summons ice to bring the sack to her.

He lets out a sound that could have been a snort. "In every Earth I've been to, men found the need to be gentlemen because they believed women to be weaker. That they must be handled with care."

It's the longest sentence he's spoken to her so far, and against her will, her lips curve into a smile.

She runs a hand over the fabric. It's soft, lightweight, and durable, and clearly not from this Earth. "So does that mean that you won't even look away?"

He doesn't reply.

"Suit yourself," she says, turning her back to him. She tosses Caitlin Snow's lab coat away and gathers her hair to one side, and then, with a brief glance at his stony gaze, she untucks the blouse. She pulls it over her head in one fluid motion. She unzips the tight little pencil skirt and lets it pool to the floor around her ankles. She daintily steps out of them, and then she slides the dress he'd given her up her legs and over her bare torso.

"A perfect fit," she says, running her hands over where the dress narrows to hug the curves of her waist. She turns to face him. "A little too perfect. How do you know my measurements?"

He does not look away, but he does not acknowledge her question, either. "You are to kill Tracy Brand tomorrow," he says. "She will be at CCU. Do not fail me."

And then he disappears.

The wind he leaves in his wake displaces Caitlin Snow's clothes on the floor, flattens them against the walls, tucks them into dark corners.

She raises a hand. Frost crawls over the grimy, blood-stained clothes.

When she balls her hand into a fist, they shatter into pieces.

ii.

It's my own desire
It's my own remorse

His interest in her is purely instrumental.

He tells himself this. He tells himself that it will be much crueler for them to be hurt by her hand than by his. He tells himself that he is only using her to bring them pain. He tells himself that she is to him as the queen is to the skilled chess player—the most powerful piece in his army, but a dispensable piece, nonetheless.

He tells himself this, but his eyes betray him.

When he watches her, he watches her not like a strategist moving pieces on a board; no, he watches her with hungry eyes. Her power is intoxicating. It's raw and untrained, but so palpable that when she summons her frost, the air thrums with it. The temperature drops when she walks into a room. The ground she treads on freezes. Any liquid around her coalesces into ice as clear as glass.

He'd watched Caitlin Snow valiantly try to contain Killer Frost, but this sheer volume of power was never meant to be contained; it even leaked from her now, when she was not consciously wielding it. He had freed her, and she now she thrives. She flourishes. And she is even more powerful than he'd last remembered—because, even if he'd already been through all this before, there are always little details like this, like her, that manage to escape the prison of time.

He watches her now, from afar. She is looking at her hands. She has been staring at them for a long time.

No doubt Caitlin Snow is still struggling within her; no doubt she has not completely separated herself from her. No matter. It will take time. Once she makes a kill, Caitlin Snow will forever be silenced.

He sees her glance at the metal handrail. She lifts her hand. It turns into ice. She makes a fist, and cracks crisscross it, until it splinters and disintegrates into thin air.

She smiles a slow, feral smile.

iii.

Help me to decide
Help me make the most of freedom
And of pleasure
Nothing ever lasts forever
Everybody wants to rule the world

She fails to make the kill.

She fails to make the kill.

She is not afraid of Savitar's wrath, but she cannot bear her own shame.

"Maybe there is more of Caitlin Snow in you than you led me to believe," he'd told her. His suit had still been locked in, his mask shuttered over his eyes.

She'd willed herself to be as unreadable as he, so she'd kept her expression blank. The only indication of weakness she'd shown was to look away when he drew near.

There is no more Caitlin Snow in her. She had killed Caitlin Snow. Caitlin Snow is her first kill. But then, why the hesitation? Why had she run? It would have been so easy—pierce Barry's leg, freeze Cisco to the floor, pin HR to the wall. She'd rehearsed it in her mind the night before. She knew her game plan by heart.

Now, Savitar takes a step back from her. "Perhaps you have never used your powers with the intent to kill."

"What are you insinuating?" she snaps. "That I can never kill them?"

"Not without practice." He crosses his arms. "Come. Fight me."

She glares at him. "Don't order me around."

"You can't, can you?" he taunts. "Because deep inside, Caitlin Snow knows that I'm still Barry Allen."

"I fought Barry Allen," she grits out. "I injured him. I made sure it was painful."

"But you cannot kill him."

"Don't waste my time," she said. Ice swirls around her hands, and the frost crawls over the ground beneath her feet, worms into the cracks in the concrete, slithers over the rough surface. "If you wanted me to kill you, you should've said so from the start."

And then she lunges at him.

He meets her head-on and knocks her to the ground.

The impact to her stomach has her on her knees, gasping for air.

She gives him a venomous look, and then she coats the entire room in ice. She seals all the exits. She conjures icicles on them a foot long, so he would shred his insides if he even dared to phase out.

She finishes this in five seconds.

He stays rooted to the spot, unmoving, watching her with the same air of disinterest.

Is he mocking her?

She narrows her eyes at him, and with her remaining strength, she traps him in a circle of jagged icicles. She lifts her hand. Ice coats the surface of his armor. She will break him open, she tells herself. She will destroy him. She will tear him apart limb for limb and then vanish him to vapor—

But even when she closes her hand into a fist and digs in her nails into her palm, even when she draws blood from the flesh of her hand, she cannot break him.

In the next second, he phases out of her trap, and he stands before her, armor unscratched and gleaming. "Good," he says. "Had I been Barry, you would have already killed me. Now kill the girl."

And then he vanishes.

She uncurls her hand.

She hadn't coated the walls in icicles, she realizes belatedly. He'd phased out of the walls.

Her wound closes. She smirks.

"Well played, Savitar," she murmurs. "Well played."

Yes, she is ready.

. . .

But, later, when she would come face to face with Cisco, she would realize that she could never kill him. Cisco, who'd watched movies with me every night after Ronnie's disappearance, and then after his death. Cisco, who'd held my hair back while I'd vomited in dingy bar restrooms. Cisco, my best friend through thick and thin. He only had to shout Caitlin's name, and her powers would be swallowed back by that yawning chasm inside her, where Caitlin Snow's consciousness resides.

Before she blacks out, she would realize that she had referred to Cisco as my best friend.

iv.

There's a room where the light won't find you
Holding hands while the walls come tumbling down
When they do, I'll be right behind you
So glad we've almost made it
So sad we had to fade it
Everybody wants to rule the world

He sees her hesitate.

It's only for a moment, but to him, that moment is an eternity.

He sees it, and he knows that she's going to retreat. This would have been the second time she'd failed him. And her task had been so easy, too—she merely had to conjure an icicle from the ground to spear into Tracy's body—but it seems that she's more intent on proving to herself that she could hurt her friends without feeling any of Caitlin Snow's remorse.

He does not tolerate failure. He's had years to perfect his plans, after all, and even if he has experienced this all before, he wants to do things a little differently this time. He'd told himself that if she fails him a second time, he would dispose of her himself.

Yet when he sees that they are about to take her away, he is consumed by a sudden icy rage. They can't take her away—she is his now, and her place is by his side. They. Cannot. Take. Her.

He is at the scene in the next second. He growls out a threat, and even as he does so he has to restrain himself from ripping all their throats from their bodies.

Her motionless form also stills his hand.

He gathers her limp body carefully into his arms. He tucks her head into his chest to shield her from the air pressure, and then he takes her back to their dingy hideout.

When he passes the doorway, he slows. He places her on a metal bed.

He folds into a crouch, disassembles his armor, and emerges from it a man.

With his human hands, he touches her wrist and searches for a pulse.

He's seen himself go through these motions already, but he doesn't expect to still feel the fear that rises in his chest as he waits for the whisper of her heartbeat.

. . .

When she comes to, the first thing she sees is a scarred face and familiar green eyes.

And then she looks down and sees that she is strapped down to a metal table.

"Relax," he says. His voice, without the echo of his suit, sounds smaller, more intimate.

She looks up at him again, startled.

He is a man again, flesh and bone, soft and breakable. His expression is blank but his green eyes betray his undivided attention. "You were having seizures from Cisco's vibes."

The memories rush back in, and she shuts her eyes.

She hears him unclasp her restraints.

She blinks. She moves her fingers, and then her hands and legs. And then she mutely straightens into a sitting position.

She feels weak. She can hardly even support her own weight.

"Can you speak?"

She looks up at him. His human face and his deep green eyes are so achingly familiar that she feels a pang in her heart. She cannot bear to look at him when he's so painfully human.

"I couldn't sleep last night," she begins. She casts her eyes down to her hands. "I couldn't sleep last night because Caitlin Snow was worried about you."

He is silent.

"She wants to know why there's so much hatred in you. She wants to know where you'd gotten that scar, why it doesn't heal." She takes a deep breath. "She's… she's still a part of me."

He gives her a measured look.

"It doesn't matter," he finally says. "You will be rid of her eventually, as long as you stay my ally." He pauses. "Will you?"

She huffs. "Of course."

"Good." He moves closer to her. "And you will not betray me."

"No. Not as long as I'm Killer Frost."

He touches his fingers to her chin and smiles. "Good," he says again, before pulling her in for a searing kiss.

She gasps. The effect on her is instantaneous—suddenly she feels the heady taste of heat in her mouth, flooding her system, rushing through her blood; she feels her strength return, and with it a drastic drop in the temperature of the room.

She slants her mouth over his. His tongue sweeps into her mouth. She tangles her hands in his hair, and she pulls away for a second to slam him into a nearby pillar. Her blood is boiling.

She has never felt so alive.

He runs his hands down the length of her body and nips at her ear. "Someone's hungry," he rasps.

She hums. "Ravenous."

He smirks at her and flips their positions, so that now her back is against the wall and he is towering over her. She pulls him towards her by the lapels of his black jacket and wraps her legs around his waist. "Well," she says breathily, intoxicated by the energy emanating from him in waves, "at least now I know how you got my measurements."

His responding smile is sly, but he says nothing. Instead his hands leave trails of heat up her legs as they inch closer and closer to the hem of her dress.

She supposes that that is a good enough response.

. . .

They wreak havoc in the city the next day, and the day after that, and the week after that. Together are unstoppable.

Eventually, of course, Team Flash prevails. Caitlin comes to her senses, and she seals Killer Frost away in that dark place inside of herself.

But even then, Caitlin will look back those days with him, and even before she's overcome by crippling guilt and shame, she will remember that rush of exhilaration, that power thrumming in her blood, that headiness of having the entire city in the palm of her hand.