Interesting to see them mention tulpas here. It's always been a bit of a meme online, but I knew a pretty tenured hypnotist who was utterly terrified of the things. He'd seen tulpas cause irreversible psychological damage to the people who adopted them, where many people would become completely attached to a character or particular fiction in their head. This could cause them to act out involuntarily, induce constant paranoia or even completely alter their perception of reality. He figures the human brain is simply not meant for multiple inhabitants, and I'm inclined to agree. Hearing the occasional story of a person who's gotten in too far with tulpas just makes me sad, particularly because there's no real easy "fix" for it. Oftentimes, people wall themselves in to an altered reality and refuse to leave, be it by their own volition, or even another's.