I've been narced 5 or 6 times that I know of.
Only once was what I would call a "dark narc". I had experienced a freeflowing reg on the previous dive in a cold murky quarry and the second dive was with a different reg but it was the same make and model of the reg that freeflowed on the first dive. When I got down to 90' or more in the cold murky water I was feeling anxious and waiting for my reg to start freeflowing again. I suddenly thought "I want to go to the surface NOW" but I realized I was probably narced and did the old "stop, breath, think" and took control of my emotions again. I finished the dive without any issues and was even able to enjoy myself once I did the mind control thing.
The freeflow was most likely caused by me test breathing the reg in the air (9 degree temps) before descending and I made sure to not do that on the second dive.
My other narcs have been more of a slightly "buzzed" feeling, similar to drinking a glass of wine too fast. No anxiety or fears and managable. I get the feeling, say "I'm narced" to myself and am able to continue the dive. However, my deepest depth has been 123' and I'm usually only at those depths for literally a few minutes.