I had a horrible nightmare. What does it mean?

What you were going through in this dream is just another of what I like to call a R.C.S.D. which stands for Random Created Scenario Dream. It's pretty much like a fixed scenario or dream story if you will. Although there were a lot of changes in the personalities with the people you met. Things happen to change from good to horrifying. Now to finish this, this dream has no meaning at all. Again just another random created scenario dream. .