what kind of scene in what kind of book?

and scary as in torture or scary as in ghost story stuff?

Ok in a nut shell its a fantasy adventure with a teenage male protagonist. I dont want to say to much but he basically looks into a mirror and he sees a frightening evil and twisted version of himself. It gets increasingly more frightening as he gets stronger. I know what i want to happen but i need a few pointers to make as scary as possible

Figure out what he fears most, and use some of that. Whatever he's afraid of becoming, definitely use that. Use physical size and things like claws and stuff to display power. Thant kind of thing.
Also, eyes. Eyes are called the window to the soul for a reason. Creepy demon eyes are always neat.
Uhh... also... crying or drooling black blood? Idk. That's always interesting.
I hope this kind of helped? I'm really better at answering direct questions.

Well what ive got so far is its bloody (of course) and it doesnt have eyes instead this black mist/smoke billowing from them. This is "the reflections" first appearance in the series and it becomes his greatest fear. To be more specific i want advise on how to write it that makes it scary. Like verb choice what words and sentece structure makes people scared without telling im its scary i want the ready to feel the fear rather than read it.

ah
okay gimme a minute or two i'll type up some suggestions

use words that set the mood, like 'menacing' or 'intimidating' or the such. Give the reader a feel for the reflection guy.
using the five senses are key. Maybe the protagonist smells something acrid? If it speaks, maybe it speaks in a guttural tone? of course sight is key, so try color descriptions.
Don't use simple ones though. Try 'ebony' or 'pitch black' or even 'void black'. Describe the physical reflection itself, too. Is it looming? Is it beastly? Savage-looking?
Sometimes unspecific descriptions are your friend. 'the figure was hazy at first' or 'there was an unintelligible sound' Everybody fears the unknown.
Show the reader the protagonist's emotions. key words can be like 'horror-stricken', 'petrified', 'panic-stricken'.
Another tip is to build fear gradually, don't rush into it and go 'suddenly there was this thing in the mirror'
maybe it slowly manifests out of the shadows, or grows from his normal reflection.
Okay that's about all I've got.

Okay, update to that last sentence. Maybe wisps of smoke at first seem to spin from his eyes, and blood slowly begins to seep from (insert orifice)
anyways my point is go slow. Unless of course you want a kind of jumpscare type-thing

Have you read "Picture of Dorian Grey"?
It's similar in which that he repeatedly keeps looking at a self portrait and it increasingly becomes more horrifying with each look.
I'd recommend that for a basis or for like pointers.

ooh man I loved that book it's honestly one of my favorites. That's a brilliant suggestion, otamega.

Thats really helpful i know i dont have special effects at my disposal or jump scares. It is supposed to be a sudden thing though because its only in the mirror and all mirrors so its a recurring aspect of his character.

If it's sudden, use words that are striking and, well, sudden. 'out of nowhere', 'suddenly' and others. i can't think of any right now.

And mirrors are always creepy or at least they creep me out

Drawing from your own emotions and fears can help add a sense of 'realness' to the story as well.

I feel and sound like a wikihow article i'm sorry

Don't give away. Let the image hit your readers like a fucking freight train.

Thanks guys