My sister vanished. Something else came back in her place.

I’m a twin. I always have been, and I always will be.

We’ve always been close. Even when we used to fight and argue over every little thing. Sam’s my best friend, and I think she knows me better than I know myself. And I know her.

Most people can’t tell us apart; they hear ‘identical’ and don’t even bother trying. They joke and laugh about us being “mirror images” and how “impossible” it is to distinguish us. Even our parents sometimes mix us up, but to us the differences have always been obvious. You just have to look.

Her jaw is slightly rounder, my eyes more oval, and there’s a small mole beneath my left ear, but not on her.

A few months ago, my sister just disappeared. One day, she was there, behaving the same as always. The next day she was gone, vanished, without a trace.

They searched and searched, but found nothing. There wasn’t a single clue as to what had happened or where she had gone. They didn’t know whether she had left of her own accord or whether she had been taken. They didn’t know if she was dead or alive.

After a month of nothing, of “We’re so sorry” and “We’re thinking of you”, again and again until I could scream, I was lying in bed staring at the ceiling, unable to sleep as the night crept into the early hours of the morning.

I slipped out of bed and crept into Sam’s room, making sure not to disturb my parents, though from the quiet sounds of weeping downstairs, at least one of them was awake.

I entered her room for the first time since she’d gone missing to find it exactly as it had been before she’d disappeared. I perched on her bed, inexplicably anxious about disturbing anything. For a moment the silence and stillness felt peaceful. Then goosebumps erupted over my skin as I suddenly felt the sensation of being watched.

But the room was empty, I told myself, even as I felt ice trickle down my spine. I shivered and leapt up, returning to my room. No matter what I told myself, I couldn’t shake the feeling of something watching me.

I woke the next morning to my mother’s screaming and father’s footsteps pounding up the stairs.

My sister had returned.

Three weeks, six days and eighteen hours after we had last seen Sam, Mum had gone to stand in Sam’s bedroom doorway, her daily ritual, to see Sam, asleep in bed as if she’d never left.

Sam had no memory of the month she’d been gone. To her, it really was as if she’d gone to sleep that fateful night, and had woken up to Mum’s screaming and wailing, to Dad’s swearing and yelling, to me, stood stock still in the door, unable to move or speak, at the sight of Sam, my Sam.

The police were baffled – they had as much clue as the rest of us. The doctors assured us Sam was perfectly healthy and had no idea about her missing memory. Neither did the therapists or psychologists. Our parents were happy to move on, happy to accept their missing daughter had returned. And so was I, at first.

The realisation that something was wrong had happened slowly, a stuttering crawl to some sort of twisted understanding.

Whoever – whatever – had returned was not my sister, was not Sam.

Initially, I assumed the prickling on the back of my neck was the uncanny feeling of being watched I couldn’t seem to shake. Then I noticed something was wrong with Sam. Sure, she’d laugh and smile, tease and joke like usual, be sweetly patient with Dad’s fussing and Mum’s questions.

But when she – it – thought no one was looking, the smile would slide off its face, its frown would smooth out, its eyes would glaze over. It was like whatever was there was an empty shell, vacant when no one was watching.

It happened repeatedly, and each time after a few moments it’d realise I was there observing it, and she’d come alive again, an easy smile returning to her face as she asked about the gossip from our classmates or referenced an old, shared joke to try and make me laugh.

Whatever it was had my sister’s memories; no matter what I asked or alluded to she understood and answered. It knew how Sam broke her wrist six years ago, the name of our childhood dog that died when we were eight, the secrets I whispered to her and the ones she whispered back when one of us would occasionally tiptoe into the other’s room late at night and curl up in to a too small single bed, knees knocking together.

I tried to trip it up, invented fake friends or made-up anecdotes. She’d catch it each time; she’d frown and correct me or laugh and play along, making the story wilder with a wink and a grin.

Next, I realised Sam didn’t eat anymore. She was never a big eater before, but now it seemed like not a single morsel of food passed its lips. It’d push the food around her plate, cutting it up to make it seem like it was eating whilst she complimented Dad’s cooking and struck up cheery conversation with Mum to distract them, then happily volunteered to clear up to dispose of the evidence.

I bought her favourite cake and surprised her with it in front of our parents. I insisted she have the first slice, handing it to her with an innocent smile. It thanked me but refused, claiming it was too full. When I pushed, reminding it she’d never turned the cake down before, its face flashed with startling fury for a moment as its brown eyes seemed to turn black. I blinked and it was gone, but the unease stuck with me.

She graciously accepted, but I saw the brief disgust as it took a bite. As soon as it could, it escaped to the bathroom and I followed behind. I could hear the sound of retching and angry muttering.

Yesterday, I finally realised that her jaw was slightly sharper than before. Her eyes were less round. It looked like me. I recalled the feeling of being watched in her room the night before it returned it appeared in her stead, and felt a wave of nausea.

So in the evening, I padded up to her door and peeked through the ajar door to see it sitting at her dressing table as it moved her hair over her shoulder. There under its left ear, was a small mole.

My eyes moved to the mirror to see her reflection. And I saw its true form. It was my face – our face – but horrifyingly wrong.

Its eyes were sunken into its face, the iris and pupil the same indistinguishable inky black. The whites of its eyes were dry, with a horizontal yellowy-brown band running through them. Its skin was whiteish-grey, with splotches of colour like bruises. Its lips were pulled back to reveal its teeth.

I froze, stood stock still in the door, unable to move or speak, numb from terror.

Its gaze slid to me in the mirror.

I waited for it to turn and rush at me, to tear me open with its long, yellow nails shaped into sharp claws.

It didn’t.

It smiled slowly, its thin blue lips stretched obscenely over gums riddled with holes and divots. It made a sound as if humming that brought to mind nails on a chalkboard and fork tines on a plate. I turned and fled to my room, and it didn’t follow.

I scoured the Internet, looking at websites and blogs that warn of demons and possessions, of malevolent spirits that inhabit a host, but none seemed to match my situation.

So I came here, to see if I could find any answers about what my sister has become, or what has taken her place.

I haven’t confronted it since, the decaying corpse masquerading as my sister with my face. I haven’t dared to.