Talking about dark dramas, I'd say: 

Byakuyakou - the plot is very dark but I found it very interesting.

Sore demo ikite yuku - don't be discouraged by the first episode as it keeps building up

I'd say Faith,Beautiful Mind and definetly Beautiful Days..They are all online and kdramas

https://mydramalist.com/17095-haunted-campus

I enjoyed it for completely different reasons, so jokes on you if you end up watching this drama