Nightmares are something natural, they are a warning after all. Even advanced oneironauts experience nightmares from time to time. There's an infinite amount of nightmares, and they have different functions and meanings. There's ways to avoid nightmares and to scape them though. • Nightmares in non lucid dreams: If you're having a common dream (not conscious), a nightmare is there because something is wrong in real life. It could be a social matter, like a relationship or a problem with someone, or they could be warning you that something is wrong with you or someone (such as having an illness you're/they're not aware of). These warnings usually happen OUTSIDE lucid worlds or lucid dreams, because they're trying to make you aware of something by making it seem real so you realize it, if its not a frightening dream, then you won't pay too much attention to it. • Nightmares IN lucid dreams: There's two types of nightmares in lucid dreams: a) Sudden consciousness b) Always conscious The type "a" is when you're having a common dream that turns into a nightmare, but while living the nightmare, you suddenly realize its a dream (sudden consciousness) These kind of nightmares were trying to warn you about something before you disrupted it (they're like the common nightmares but that get disrupted, that's all). They still were trying to warn you of something. Try to look forward to finding out what it is. The type "b" are lucid dreams that get triggered. This happens when you say or do something you shouldn't, that shows that you're aware you're dreaming. Your mind does not want that. If you say out loud things like, "I know I'm dreaming" or "This is a dream", it could turn the dream into a nightmare (in the best of cases you'll wake up immediately). That's when a nightmare begins in a lucid dream. There could be other triggers, like breaking a rule (that is, if you have rules. In my dream worlds, I have rules that I can't break or otherwise I'd wake up/have nightmares). An example is the second rule I have in Twisted World: Never interact with non-humans, or it'd expose my identity as a non-human as well. Experienced oneironauts like me can say that they're aware they're dreaming inside lucid dreams because we're so used to having these dreams that its hard for them to turn into nightmares or waking up. (I'm gonna make a third part, I'm afraid there's little space left to write)