I provided a bit of extra information about the trips to give context, but they often seemed like one big continuous hallucination about all of reality, usually based on fear.
I have a hard time deciding between these three:
After a day of very heavy weed use - at least a gram of the very strongest bud - after I'd been smoking daily for close to half a year. All the doors looked like the faces of zombified sailors with those white hats on their heads, and I'd feel like I was travelling endlessly with them on an old boat. Then I looked up and the vent was grinning at me in a very sinister way, and then I snapped. It lasted maybe twenty seconds, my house turned into a maximum-security prison, then a mental institution with chains and crap like that, then an abandoned hospital, then a slaughterhouse, with my room being a meat locker with gruesomely mutilated corpses hanging from the ceiling. This morphing occured over roughly five seconds, then everything twisted over and over until I was travelling through spiralling structures. I heard drills, saws, and screaming from the room next to me, and I saw blood splatter on the walls as the screaming happened. I was also hearing distorted voices, which I couldn't make out. When I saw the blood splattering, everything broke apart, the walls were made out of bleeding red meat and I felt like I was dying from something like a seizure and energy was racing through and ripping everything apart (probably the beginning of a psychotic episode). I felt like I was living in a government-run concentration camp (which was supposed to be subconscious). My vision turned completely red and bloody and I heard a very disturbing hum. I lost touch with reality and and thought I was in a grey/crimson red hospital with those padded rooms, but the rooms were only visible through small round windows and they were filled with blood and ripped body parts, moving about like a huge washing machine and acting as a grinder of bodies. This was apparently where you were thrown once you died, and I was pretty sure I had died. Then everything came back together and it felt like forces were harrassing and pushing me around and my bed was shaking violently for a few minutes and slightly for the rest of the night. Weed is a hallucinogen, and it can be a fairly strong one. Based on my experience, I don't think it was laced, but it was extremely strong bud that I was coming down from, and I was pretty tired as well.
The second experience involved snorting synthetic THC (which probably worked when it dripped) and speed. Then I pulled out the salvia 20x and took a good hit from it. I waited a few minutes, did it again. Waited a bit, did it again. Each time, my room splitted into stringly beings (the salvia ones) and they basically picked me up, pulled me inside the wall, and carried me to Lady salvia or something like death, but I was way too high to be smart and stop. I think I did it four or five times that night, and I was completely blasted into oblivion. When I came down from the last salvia trip, I was flying through huge fractal structures and over mountains of coloured hexagonal crystals for a while. After a good hour, there were changing coloured streams swimming around in my vision and it was like being in a futuristic underwater world. Everything got closer and further like it was stretching, and things looked very cartoony and had these weird fast-paced animations associated with them. That night, almost anything I thought about came true. I heard a bomb dropping on my house, tornado sirens, and people breaking into my house. I hid in the bedroom, and I think I asked 'god' (not something I believe in) to spare me and end the nightmare. It ended after a long, difficult night.
The third experience occured when I was peaking on mescaline and smoked two bowls of really strong bud, and possibly had some 5-htp pre-trip. I was getting the usual, a cross between MDMA and light acid sounds roughly accurate, but I haven't tried either of those so I wouldn't know. Everything was great until I smoked. As the high came on, I yelled, almost like I was on a rollercoaster, and it felt like my body was racing or falling into the room and coloured bubbles radiated from objects as they rotated. It was a lot of fun for a few minutes, until I peaked on the weed while peaking on the mescaline.
Once again, everything started to grin at me and then twisted around like an unrealistically stretchy substance. I saw 'symbols' and etchings of demons with long, stilted, bony legs, a lot like what I've experienced on the few heavy doses of deliriants I've had (but everything else was completely different). Everything spun around me and it felt like objects were performing a sadistic ritual and celebrating my death. Tiny movements would really frighten and confuse me, it was like my vision was in murder-mode, every tiny change would feel like nails scratching a chalk board.
Then came the worst, fifteen minutes of pure terror and a few hours of residual terror. I felt like I was straying too far from normal perception and I heard sirens, saw stop signs, police lights on everything, like my own mind was telling me that I absolutely had to end this if it were to survive. I felt like I was leaving an arena of reality, and was now hovering in a field of land mines. A voice clearly told me that I was living in a secret prison, similar to the first trip I described but a lot clearer, and I saw it. My living room was imaginary, I was really in a tiny grey prison cell, the truth was that I had lived in a hallucination designed by the government to distract me from actual reality. The voice kept repeating "you're living in a secret PRISON! They know that you know!" I thought I could see shaking assassins ('executioners') behind every door and around every turn, using objects as shields as they ran around like they had strange powers and were accessing the matrix. They were shaking with the desire to "terminate" me, or basically kill me by deleting me from the program, and I could see their trembling legs behind the doors. Somehow, every door had been lifted about one foot off the ground. Sometimes it wasn't a human assassin, it was an extremely creepy and retro Android. My adrenaline was pumping crazily for the first fifteen minutes and I began to cry in fear and ask myself why I had ever tried drugs. The feeling faded over a few hours, but I can often tell that it's there, especially when I smoke, which is partially the reason for why I stopped.
And there it is. I'm not sure which of those hallucinations was the worst.