Descending into Gloom’s Lair from Tears Of The Kingdom. Not sure about long form but I love the parts with the choir. Absolutely sounds like you’re entering a place that you shouldn’t, but then the music changes to sound more heroic the longer it goes on
Requiem for a Dream’s main theme
Ministry - Khyber Pass
And while I’m at it, Tool - Legion Inoculant
Absolutely lives up to its title.
There is a good variety of dark ambient music at Cryochamber on Bandcamp.
Boris - Dronevil https://www.youtube.com/watch?v=BjRS01JApRs
Offermose has made a great album called Stilhedens Tårn. It’s some kind of darkwave/ambient/dungeon synth.
Lord of the Rings soundtrack has some good suspense music
Threnody to the Victims of Hiroshima - Krzysztof Penderecki
10 minutes of eerie, menacing beauty.
Erm not exactly pleasant IMO, but definitely menacing! It’s amazing how creepy string instruments can sound.
Orbital - The Box 6min album version
Just found a 28min The Box ‘full version’ too, not sure what is going on there, album version is decent.
The whole Orbital - Insides album has a gritty mood.
What style do you like?
I think dark synthwave generally fits that description.
Bandcamp is your friend. Check out an artist called ‘The Secession’. You can also check him out on the Youtubs.