What is an Existential Nightmare

Question

please give examples in depth

Answer 1

It is the most extreme example of doubting or believing in one's own existence, existential anyway.  

The nightmare part makes it doubting definitely.  For example, if aliens came down on a spaceship and said that they invented us and we were no more than bugs, one could have an existential nightmare in front of them because everything they saw as concrete, real, and alive was really false, abstract, and fake.