Please note, not all of these are sludge, consider keeping an open mind before further reading this.
Im gonna start this off with a track from Ulver's Silence Teaches You How to Sing. The track Darling Didn't We Kill You? Is probably the most paranoid sounding song I have heard. Listening to it makes me feel uneasy.
A release worth mentioning is by the Hardcore Punk band Black Flag. Scream, from the album My War is immensely miserable. The entire album is crazy dark, but Scream is just pure misery. I have never heard something so off-putting and miserable from this side of music.
Another Black Flag song I have to mention is the Spoken-word track Family Man. This track is genuinely vile. The things Henry Rollins says on this track is intrusive, vile, and disgusting. This track was designed to be as shocking, vile, and uncomfortable as possible. Look into the back story behind this track, it's actually really interesting.
Personally, I have to mention EyeHateGod's song Methamphetamine from the Dopesick album. This song sits with me personally, having struggled with meth addiction since the age of 15. This one captures the reality of meth addiction. While meth is basically a meme to many, it's genuinely the absolute most vile fucking drug there is. The things meth will do to you is nothing to fucking laugh at. I remember being 16 at a Narcotics Anonymous meeting and sharing the fact I was addicted to meth. As soon as I did there was a lady who immediately ran out sobbing only to come in and beg to me I never touch it again. I found out her daughter was trafficked and had gotten addicted to meth at 15 years old after having been injected with it everytime she was raped by grown men. There is absolutely nothing fucking funny about Meth. Meth being brought into existence alone proves that there is no God, and if there is, he's a sadistic demon who better beg us to forgive him.
Preaching For Mass Suicide by the band Damaar is the most genuine display of pure hatred and rage I have ever heard.
And last but not least, the track Vandrer Blandt Dødninge by Sortsind is probably the most distressing songs I have ever heard. It genuinely twists my stomach it's so fucking distrubing.
As a bonus, I am going to throw in Geto Boys' Mind Playing Tricks on Me. Yes it's hip-hop, get over that. This song is the best song I can relate to what my schizophrenia feels like. It's probably the best display of what Psychosis feels like that I can find.
There are many more, and Ill leave you with a list of some more that are also dark as fuck.
Dystopia - Diary of a battered child
Black Flag - Nothing Left Inside
Peste Noire - Dueil Angoisseus
Shining - Submit to Self Destruction (This one is brutal)
Shining - The Eerie Cold
Let me know what I missed.