In retrospect, it wasn't the wisest final sentence to yell at my boyfriend as I kicked him out of the flat:

"If I ever see you or that…" I floundered for the right word, "hussy again, I'll kill you both."

The front door slammed behind him, a whirlwind of disturbed particles danced in his wake, illuminated by the nearby desk lamp. They began to settle as his footsteps receded, my shaking hand pressed hard against the back of the door. The silence grew until it filled the space between every piece of furniture that we'd bought together. Total silence, save for the hum of the fridge from the kitchen behind me and the occasional burst of canned laughter filtering in from Old Mrs Haggerty's television downstairs.

The unfamiliar emptiness was oppressive. Overbearing. Interspersed only by the dark energy that consumed me.

I thumbed the lock and pushed away. Paced the flat, crashed some cupboards in search of a wine glass, poured myself an unhealthy slug of Syrah, and downed it. Then another. I stood there gripping the kitchen countertop until my fingers were numb. Pushed away and paced some more. Then yanked all the photos of Steve and me from the lounge sideboard to dump them in the wastebasket, and ultimately slumped, seething, on the edge of our bed. My bed. Not ours.

Cheryl, predictably, blew up my phone trying to justify her actions, using every cliché imaginable.

'We didn't mean to.'

'One thing led to another.'

'It was just sex.'

I ignored them. Changed her contact name to bitchtits, which made me feel a whole lot better. Or not. Quite what he saw in her was baffling, besides the obvious pair of physical attributes shamelessly flaunted in low-cut tops around the office. She had a face like a scrunched-up poly pocket and would rather file a health and safety complaint than piss on someone if they were on fire.

Her final message before I blocked her was to 'talk about it' at the party. The company Halloween party I'd been looking forward to for months. They'd both be there. Probably all over one another. I'd avoid them as best I could, inevitably getting cornered by her at the punch bowl.

"Hi, Lilly."

I'd cast an icy glare. "Hi, Cheryl."

"Sorry about Steve."

"Me too, you fucking bitch."

"Don't—"

"Don't what, Cheryl? Don't worry? Don't pretend I didn't love him? Don't wish you both could have had a modicum of humanity and kept it in your pants? Jesus."

"I'm sorry."

I'd scoff at her insincerity. Say nothing and seethe.

She'd probably gloat. It's her thing. "Perhaps you're just not as… exciting as he needs."

Throwing my drink in her face and smashing her head into one of the pumpkins decorating the trestle table of nibbles would be oddly cathartic.

I let the visions fade and flopped back on the bed, wisps of burnt oak hair tickling my cheek as it pooled around me on the pillow. I rolled my head and cast a wistful gaze to the velour costume hanging on the back of the door. Seriously considered not going. What use is a witch without her accompanying warlock?

Even if it hadn't been too late to return it, the lady with the straggly grey hair at the hire shop had said no refunds. Kind of strange, but then the whole experience had been off-kilter. She'd been a little cagey – almost reticent – to let me rent it. I’d spied it wedged on the end of a dusty rail, plucked it down, held it against me, beamed, and tried it on in the changing room. Such a perfect fit, like it was made to measure. So I took the plunge. Impulsive, which is not normally me.

At the cash desk, she fixed me with a piercing stare over the rims of her octagonal glasses. A chill ran up my spine as she tried to talk me out of it, offering alternative costumes that weren't quite as pretty, nor as dazzling. I wasn't swayed. If anything, the more she tried to put me off, the more I wanted it until she eventually relented. As she rang up the total, I was unsure if "Just be careful" was a standard sentiment to remind me not to damage the garment or some sort of warning.

I would have given her the benefit of the doubt except for two actions. Firstly, she put on disposable latex gloves before touching the costume and wore them throughout the transaction. She only removed them when I took the dress from her and folded it into my bag.

And as if that wasn't bizarre enough, from beneath the counter she then retrieved a dented metal tin with a hexagram stamped into its lid. She dropped the gloves inside, fiddled with a box of matches, struck one, and tossed it in afterward, sealing the burning glow with the lid. Smoke wafted from beneath the rim, filling the immediate space with an acrid stench that she waved away before replacing the box.

She didn't wear any gloves for the guy after me who wanted white Bee Gees flares and a flower power shirt. Maybe the seventies were a safer era.

Odd proprietary practices aside, the dress looked amazing on. It hugged my frame without emphasising the admittedly curvier-than-I'd-like evidence of recent chocolate indulgences. The purple and black panels flowed effortlessly around me and I'd been aching to show it off at the party.

It seemed a waste of money not to show up. Plus, there was a chance I'd meet a less two-timing warlock. That dishy guy from IT would be there. Playing tonsil hockey with him would show stupid and stupider I'm not bothered, and that they're better off with one another.

I bristled. The costume swayed, catching my eye and I focused on the way it spun and danced as if it had caught a stray breeze despite the windows being closed to keep out the frosty evening.

Watching a moment longer as the material billowed, settled, and then remained dormant, my thoughts wandered again. I hadn't anticipated doubt surfacing but the imagined conversation with Cheryl had sparked something.

What if I drove him away? Had I been too inattentive? Not put out enough? If I was being honest, Steve wasn't all that exciting in the bedroom, but maybe it was me all along? Work's been pretty full-on and I've been more distracted lately.

I swallowed and pushed the self-deprecation away. Fumed at dark images of them together, doing all the things we did. Autumn walks kicking leaves. Sharing ice cream with a single spoon. Cringing at dreadful B-movies on Sunday afternoons.

No, I scolded myself. It's not me, it's them.

The dress billowed again and I eyed it flitting and twitching, almost with a mind of its own until it eventually stilled.

Rolling upright, I stood, crossed the room, and unhooked the garment from the door. Returned to the mirror. Held it against me, twisting this way and that, appraising. Such a perfect costume.

I deserve some fun. Maybe this'll be the opportunity for a fresh start.

Stripping off my top and jeans, I slipped the dress over my head and adjusted it. Smoothed the satin detail and twirled. Loved the weight of it splaying then resting against my calves. So pretty.

Yeah. I'll show them how tough I am.

I focused hard on my reflection. "Screw them."

A fierce wind whipped through the flat, scattering bills I hadn't yet filed on the dressing table. I stepped back and the lights flickered, fizzing then stabilising, a harsh yet indiscernible whisper carried on the tails of the gust swirling me. The cyclone abated, strewn papers fluttering to rest.

An inky shape shifted in my periphery and I snapped my head to catch it.

Nothing.

Returning to face the mirror, I peered at myself in the dress. The edges of my reflection were fuzzy, like I'd been cut out using crocodile scissors. I gently tilted sideways, my likeness woozily following, as if trying to catch up.

What the…

I took another step back, my calves bumping against the footboard. A faint aura of burnt toast drifted. Staring ahead, I swear my reflection smirked. Couldn’t be sure, but my pulse quickened all the same.

Three sharp, measured raps on the window seized my attention across the room. My heart pounded. Unmistakable bone on glass. All the horror movies I'd watched screamed inside my head not to investigate as I focused on the source. I knew I should walk away. Run. But curiosity drew me to the window.

Ten feet… Six… Two.

Tucking a stray lock of hair behind my ear, I reached out and bunched the central edge of burgundy fabric in my fingertips, counted to three, took a deep breath, and yanked it aside.

Nothing.

Just the moon on the horizon, bleeding an orangey glow across its surface from the recently departed sun.

I stared at my reflection in the window. Waved my free hand left and right. Its motion tracked perfectly, a silhouette against the eerie backdrop of the large moon.

Dropping my guard I almost laughed out loud at my ridiculous behaviour, ready to tell myself to get a grip when an icy blast of air raced past me, as if the pane was missing. I released the curtain, stumbling backwards.

Another whisper circled. This time it was clearer, drawn out. Breathy. It was my name, eddying my head. Like cinematic surround sound.

Fear gripped me. Rooted me to the spot.

As abruptly as the rush had started, it ceased. The curtain swayed then settled.

That's when the knocking resumed. Metronomic. Unhurried. Persistent.

Knock.

Knock.

Knock.

Rotating my head to locate the direction of the source, I paused. Waited. Stepped into the centre of the room, portraying outward calm while my insides were roiling, intent on making a run for it if the need arose. To catch whatever this was off-guard. Make it to the front door. Shit. Why had I locked it? Stupid of me.

As I passed the mirror, I caught my fractured reflection, twisted and gnarled.

Knock.

Knock.

Knock.

My face – or at least the face that was an approximation of my own – jittered, like an untuned TV station. It flickered randomly from colour to black and white, just for fractions of a second. One moment it was me, the next I had a hollow expression with makeup streaming down my face, then it was me again. All in the blink of an eye. I couldn't tear my gaze from it.

Another flash. This time I had cadaverous cheeks. A shock of wiry grey hair like the lady at the hire shop. Wrinkled skin as if I was well over a hundred, and only half my teeth that I bared in a maniacal sneer, before the image flickered back to my jagged reflection.

I should have run but I was glued to the horror ahead of me.

Knock.

Knock.

Knock.

There was no doubt. It originated there.

Inside the mirror.

My visage flashed again. Lifeless, sunken eyes. Me… sort of. There was the briefest glimpse of something shiny in my hand, alongside the costume. I gasped at the screwdriver streaked with dripping blood.

Knock.

I glanced down. Red splatters peppered the cream carpet by my feet.

Knock.

One hand flew to cover my mouth. The reflection didn't follow suit. A broad grin broke out on my – her – face. The colours of the dress splintered. Switched purple and black for a split second. Then again, for a little longer.

Knock.

She reached out an emaciated hand, turned it palm up and the wind howled, stronger, whipping through the flat. Bottles tumbled from the dresser. The Parisian watercolour above the chest of drawers tilted off-axis, pulled taut on its wire and was yanked from the hook, smashing to the floor.

The costume swirled and I fought the invisible force tugging me towards the mirror. Inch by inch, no matter how much I screamed and panicked, I was dragged.

As my fingertips brushed the glass, I scrabbled against it, but there was nothing to offer resistance. The surface was liquid.

Elastic.

Timeless.

I spun. Clawed for the bed frame, only managing to snag the corner of the duvet, dragging it partially off before I lost my grip.

Moment by moment, the mirror claimed my struggling form, until I was sucked through completely.

The vortex spun down.

Quiet restored.

Nothingness swamped my senses. A lack of absolutely anything. More complete than earlier. No fridge. No TV spill from downstairs. An enormity that was almost claustrophobic in its void. My shallow breathing didn't occupy the space; it was amplified inside my head. I tried to find my voice but it cracked. Didn't seem to have any presence when I called out. As if invisible walls of impenetrable cotton wool absorbed every sound.

And yet, ahead of me, a rectangle of light.

I gazed back into my bedroom. Standing in front of the rumpled bed staring at me was my twisted reflection. Her piercing green eyes shone from between the curtain of hair, and a lopsided grin formed.

She turned and paced out of sight towards the kitchen.

I stepped forward. Hammered on the glass; whether for attention or to break it made no difference. Nobody came.

Scrabbling around the edges of the mirror, I fumbled in vain for a secret switch or trapdoor or… anything. Clearly whatever had escaped had found a way to knock and be heard. Maybe I could do the same.

I banged on the glass in frustration and it barely flexed.

Again and again, I battered its surface until my palms ached. Called out. Screamed.

Nothing.

Then the tiniest sound. Distant, yet audible, filtered into the space.

I paused. Leaned into the edge of the glass and pressed my ear to it.

Drawers opened and closed. Rummaging from the adjacent kitchen. Utensils and tools jangled. Then stillness.

The thumping heart inside my chest was the only proof I existed, elevated breathing swallowed by the inky abyss.

My doppelgänger sashayed into view, screwdriver in hand, and I stumbled backwards. Stared. She picked up the witches' hat to complement the dress and stood in front of the mirror, adjusting everything for the perfect angle.

Her lips moved and two words filtered through the mirror. My words. Contorted yet discernible. "Screw them."

From behind my glass cocoon, I shook my head, panic rising. I banged and hollered to no effect as she delivered an impish grin, slid the screwdriver into her sleeve, and headed for the front door.

In the silence of the flat, the latch clicked shut behind her.