#1
I am making a concept album LP thing with my band. We have run into a problem. We are trying to make one specific song have a sinister sound, with a sense of doom. We want a really huge sound. Our problem is whenever we try to make it sound more sinister it ends up sounding too much like a metal song. We were aiming for like a cross between Pink Floyd's "The Trial" (https://youtu.be/FCMHmDnfD6I?t=4m15s) and the end of the Beatles' "I Want You (She's So heavy)" (https://youtu.be/n4vHWnTMc9Y?t=6m30s)

I need tips for writing this song. Anything will be helpful.
#2
The most sinister albums i know of are.

Bohren - geisterfaust
Actually nothings more sinister than that..l

Keep it slow, be unpredictable (random jarring stops) odd sounds.
Go out of key, major chords in the right/wrong place can really work wonders.
Also use diminished and augmented chords and scales.
A repeated phrase can be twisted if you do it over and over, then change it when you have created a false sense of security.
A sudden blast in a soothing quiet interlude can take on the characteristics of a jump scare.

Tupelo by sikth is a good one too.
And dont forget the imperfect cadence.
Try listening to and with her came the birds
#3
Oh and...
Never disclose the horror, lyrically be as obtuse as possible. In a horror movie when you finally see the monster or whatever all that suspense fizzles away.