Here are my thoughts on the story:
To me, it seems to be lacking a certain something. Perhaps, it's in the build-up. I do like that this disembodied torturer with its arms reaching and messing with the character but I wish we got more incidents that increase interaction with the protagonist. Build upon it though to really sell its presence: does smell when it appears, how does it voice sound when talks (guttural as if backed up with phlegm, etc). Really try to sell it this thing as something you wouldn't want to be in a run at night with.
I think more interaction might shed a little more insight on the "creature" --without completely divulging what it is-- but rather its intentions. I like mystery to a monster but it almost seems too ambiguous to the point that it doesn't really make sense. It's just happening and that's it.
The ending could work but it didn't feel like we really achieved that true state-of-terror for the character you were trying to relay because it felt rushed. I think with more incidents sprinkled throughout a set time-period (say, a whole year, for example) and then have whatever action that makes the character overcome and believe the ordeal is over, followed by the five-year time-skip, would really sell the ending.
As it stands, the story feels like it needs to be fleshed a bit more, so we can really connect with the character and find the situation frightening and/or frustrating until the end. Maybe, add involvement with the family some more too.
Hope this helps!
The next part is dedicated to some errors I noticed throughout. I made corrections and annotated them in :
When I was little [remove *I*] every night I would have this dream. It always felt so lucid and I would never wake up in my bed. My five-year-old brain could never comprehend the primal fear it was experiencing. My twin sister never had this dream [,] only me. We used to share a room, her bed beside the window, mine at the wall. They put me on medication but that only made it worse.
The dream always [started] with me “waking up” in bed in the middle of the night. It would be so quiet that I thought I had gone deaf, until the whispering started. Slowly the whispering would get louder and the walls would begin to slowly melt, sticky black arms slowly growing, reaching out towards me. And suddenly everything had a slow [,] stuffy feeling, like a dark magenta tint to the twisted reality.
I would scream, but no sound would come out. I would try and try but I could never be able to. It felt like I had no lungs whatsoever. Immediately after I [started] screaming, almost like a trigger, the arms would begin to shoot out towards me. I would scramble [out] my bed as fast as I could, but my body felt heavy. Just fast enough to evade the arms but not fast enough to be able escape unscathed.
After leaving my bed the whole room would suddenly start to melt into the same black sticky substance as the arms. Slowly [,] it would collapse onto me [;] most of the time [,] this would be the end of the dream. I would wake up in a cold sweat on the floor crying and screaming. My parents would come in and reassure me it wasn’t real, all just a night terror.
A few years passed and [I] changed medication and I stopped having the dream. Until one night when I was ten years old. This [was] after my sister and I had separate rooms. We had [gotten] my [daytime] medication dosage wrong and I was having lucid hallucinations. The first few hours I kept hearing my name being whispered by a voice that felt so real. Every time this voice said my name it sent a chill down my spine. But that was it, until one point, around midnight, the voice laughed a disturbing terrifying laugh.
Then it all kicked off [:] the walls began melting into the arms. This time [,] I didn’t scream, knowing that it would just make things worse, I just slid off my bed and ran. I could barely walk [; my] body felt so heavy, I thought the room would collapse on me like it used to. But I managed to make it out relatively unhurt. However [,] it didn’t end there because the voice was back.
“[Oh,] so you want to play games [,] little dolly?” Another spine-tingling laugh came from seemingly nowhere [.] “I’ll hide [,] you seek!”
Terrified [,] I ran through the hallway, the walls slowly beginning to melt into the black molasses. I got to the end of the hallway, only for the door to [open-up] to the hallway again, seemingly endless. That’s when I saw it. The light was on in the bathroom. Cautiously [,] I wandered in, but as soon as I entered [,] the light was turned off. I heard a little chuckle. It was coming from outside.
“[You] can’t find what you can’t see!” it sang. It was right outside. “I’ll wait here quietly, [‘cause] you’ll never find me!” It was taunting me. I stared into the mirror, and watched my pupils dilate for an hour hoping it would go away.
After an hour [,] I sighed in relief[;] it was gone. I decided it was time to go back to bed. I took one last look in the mirror, and saw a purplish black figure standing behind me.
“THAT’S NOT HOW YOU PLAY!” it screamed. It lunged at me and all I could do was watch . That’s all I remember because I think I passed out after the creature lunged at me. I woke up in a cold sweat on the bathroom floor. Dragging myself up, I stumbled into my room and curled up into a tight ball and cried.
Five years later [,] I’ve started hearing the voice again. In a low whisper and distanced far apart. Its calling my name. [It] says to come play. Last night [,] it said something that sent a wave of terror into me.
“[This] time…… you hide and I’ll seek.”