Years ago I read a book (read softcover in 1990, I think -- somewhere between 1987-1991, but I don't know how old the book was it wasn't a old cover, so I assume the copy was published in the 1980s) about this guy who had to tap into an AI (artificial?) brain, because the brain controlled/created a fantasy world, but started making the world lethal to users. And I think people who went in couldn't get their consciousness out.
Like I said, it was 30 years ago. But I don't really remember any cults or outside influences and I think the powers controlling the brain shut down access. I believe the brain was artificial and possibly giant brain. it was created to host the fantasy world, but the brain started making adventure in the AI potentially Lethal. I think the hero of the story had something to do with the brain or a lot of experience in AI, I don't remember.
I remember one of people in the fantasy world (I think he was a programmer or at least knew the hero), whose body died still had consciousness in the brain (I believe he was a wizard in the story)
Any ideas about the name or author of this book?