@wyld-of-wonderland
Here's some stuff:
Yomawari Sensei - based on real stories
Yamikin Ushijima-kun - just what description says - its dark
Aku no Kyouten - psychological thriller, a psychopath narcissist perfectionist doing his routine things, ending might disappoint you
Arigatou - weird and increadible
Endo Hiroki Tanpenshuu - also check out
endo hirokis's eden, a literal magnum opus, but requires some patience
Eternal Sabbath - psychological thriller with a really strong plot
Sekai no Owari to Yoakemae /
Hikari no Machi - early inio asano's coming-of-age things
Hitsuji no Uta - some kind of modern vampire drama? im just a tome kei fan whatever
Kokou no Hito - yet again a psychological thriller, about a man's development mostly, first volume looks like a shounen tho, many deaths involved
Skyhigh - posthumous drama?
The World Is Mine - becoming a monster to just survive a day longer, weird ending, a great trainwreck
Berserk - golden age arc is pretty relevant, everything else is not, its a classic anyway
Kono Sekai no Owari e no Tabi - as a bonus, pure surrealism