A fiend or incubus formerly supposed to cause trouble in sleep.
A condition in sleep usually caused by improper eating or by digestive or nervous troubles, and characterized by a sense of extreme uneasiness or discomfort (as of weight on the chest or stomach, impossibility of motion or speech, etc.), or by frightful or oppressive dreams, from which one wakes after extreme anxiety, in a troubled state of mind; incubus.
Hence, any overwhelming, oppressive, or stupefying influence.
The schizophrenic mind is not so much split as shattered. I like to say schizophrenia is like a waking nightmare. Elyn Saks
A person can only be born in one place. However, he may die several times elsewhere: in the exiles and prisons, and in a homeland transformed by the occupation and oppression into a nightmare. Mahmoud Darwish
It's hard finding people you trust or who aren't going to take your money. Everyone wants to get a piece of whatever you're doing. It's a nightmare. Cara Delevingne