I will also have to agree with diminished 7ths and 9ths. I am particularly fond of them as arpeggios and with a few strings (or even all the strings if you feel like it) slightly out of tune. Creates a very unsettling sound. Seems to be a good go-to for what I would classify as “dark” and I personally find doing so to be quite satisfying, and even highly entertaining, for whatever reason. 🙂