4 - Major
Clinical Ambient, Deathdream, Isolationism, Ritual Ambient
Arctic Ambient, Atmospheric Speedcore, Blackened Speedcore, Darkstep, Doomcore, Minatory, Flashcore, Slambient
Outside of the communities that listen to it, I think there's a bit of a misunderstanding among people that ambient music is all new age "heal your soul"-type music and background audio for spas. And while there's definitely been a level of new age BS that has inhabited part of the community since the genre's inception, and its usage as mood music is matched in quantity by few genres, ambient has always been about one simple thing: evoking atmosphere. And while that purpose can be applied to align your chakras or purify your spirit (or whatever other post-World Music exoticist bullshit someone is disguising as genuine folk healing and selling you) or to get you in the mood to buy some overpriced candles, it can also be applied in a variety of directions, including some that are more... sinister.
Dark ambient has its origins in the Industrial movement of the 70s and 80s. Pioneered by industrial forebearers Throbbing Gristle (particularly with their soundtrack work for films such as After Cease To Exist and In the Shadow of the Sun) and influenced by the space ambient music of the early 70s, the genre was quickly picked up by a new wave of industrial artists, with influential early records from :zoviet*france: and Lustmord propelling their creators into long and fruitful careers in the dark arts of ambient industrial. The 90s was a period of change for the genre, bringing with it not only a new name, dark ambient (termed to describe the music of one Peter Andersson AKA raison d'être), but a significant increase in prominence as dark ambient not only became the genre of choice for the horror-tinged soundtracks of games like Silent Hill, Fallout, and Resident Evil 2, but also a more fully-fledged part of the ambient community in its own right, with Lustmord in particular reaping the benefits of several groundbreaking records, including Stalker, a collaboration with ambient veteran Robert Rich.
The genre has largely continued a similar trajectory since the 90s, playing a prominent role in all kinds of soundtracks for games, TV shows, and films while cultivating a significant scene of its own. Perhaps the biggest dark ambient success story of the 21st century is Leyland Kirby, whose manipulations of vintage jazz records to create eerie soundscapes under the name The Caretaker has drawn a lot of attention, especially with the release of his six part concept record Everywhere at the End of Time. As with most ambient styles, dark ambient has seen a significant rise in output with the advent of Bandcamp.
J. Greinke - "Moving Through Fog" from Cities In Fog (1985).
Robert Rich & Lustmord - "Hidden Refuge" from Stalker (1995).
raison d'être - "The Hidden Hallows" from The Empty Hollow Unfolds (2000).
Tor Lundvall - "Platform #3" from Empty City (2006).
Andrew Hulshult - "Embrace the Darkness" from DUSK (Original Game Soundtrack) (2018).