Franchise context: The write-up below is from the parent franchise article (anime, book series, or IP) rather than the card game itself. A dedicated CCG profile has not been written yet.
A nightmare, also known as a bad dream, is an unpleasant dream that can cause a strong emotional response from the mind, typically fear but also despair, anxiety, disgust or sadness. The dream may contain situations of discomfort, psychological or physical terror, or panic. After a nightmare, a person will often awaken in a state of distress and may be unable to return to sleep for a short period of time. Recurrent nightmares may require medical help, as they can interfere with sleeping patterns and cause insomnia.
Source: en.wikipedia.org/wiki/Nightmare