"Dark" is kind of subjective, but I guess I know what you mean. Substances that I've had difficulty integrating with, or which produced an eerie state:
3-MeO-PCP: as mentioned before, this can be ridiculously good, but can go ridiculously bad if too much is taken. Someone said it before: schizophrenia. My worst ever experience (which I've mentioned 100 times on this board) was a stupid combo of stuff taken throughout the day, the catalyst was re-dosing 3-PCP and taking a hit of JWH 018.
My psyche divded into parts which then engaged in a discussion with each other about what to do with "him" (him being me, who had become an observer without control of thoughts of body). I heard my parts speaking just as clearly as people in the room. They argued about whether or not "he" was having a panic attack. During this stage I had a total lack of feeling, being detached completely. It was that numb feeling while my mind's splintered aspects conversed that scared me to death, because I knew something HUGE was coming. One of the aspects kept trying to deny it: "No he's not, he's fine.. he's fine.... he's going to be OK" but it was getting shaky and desperate. Another aspect told me very calmly but VERY forcefully to go and get the stash of clonazepam and chow down.
I made it to the stash, knocking over everything on the way. My body was shaking so hard I could barely pick up the pill. In fact, I knocked the bag and all the other things I had stashed and they went all over the kitchen floor. I was on my hands and knees grabbing anything white (the clonaz) and shoving them into my mouth and... rubbing them into my left eye.... yes, I know...
I don't remember much more of that. I came to standing outside in the rain, still shaking, and wondering who I was.
Obviously that was a total fucking disaster and entirely my fault for using shit throughout the day (MXE, 3-PCP, a-PVP). I got my punishment, don't worry. Never been so terrified in my whole life.
Briefly: I have smoked d.metel (datura with purple flowers - possibly the most beautiful plant on earth) numerous times in very small quantities, and it certainly had some interesting effects. Improved respiration, increased energy, alteration of light perception, seeing light in the dark (hard to explain that one, but when I was in bed and the lights were off - I could "see" the darkness, it was illuminated from within).
All good or interesting experiences. But it must be known that I grew the plant from seed, tended it with great care. Took pieces of the flower after it had wilted, dried the pieces, and then rolled them up with normal tobacco into a cigarette. I never put in more than 3x3cm of the flower. It is very, very powerful, even in tiny amounts.