The void. I experienced it a few years ago on one of my first (almost) full dose acid trips. It was complete confusion mixed with a bad feeling. It basically felt like I had lost all hope. Nothing, it seemed, could have made me feel any better at that time. It lasted for about 30-45 minutes and started about an hour after I ate the hits.
Not really any visuals at that time, more of a fading away/almost black out type visual thing going on. With a black and white checkered grid theme. I think it may have been a low blood sugar episode, now that I think about it. Mixed with a strong comeup on some potent l and i was not good.
After it went away and I regained some control, I felt great.