incubus

plural incubi · plural incubuses

1. mediaeval folkloreAn evil spirit supposed to oppress people while asleep, especially to have sex with women as they sleep.
2. A feeling of oppression during sleep, sleep paralysis; night terrors, a nightmare.

   it increaseth fearful dreams, incubus, night-walking, crying out, and much unquietness […] .

   Burton with W.H. Gass, The Anatomy of Melancholy, NYRB Classics ser. (New York: New York Review Books, 2001, orig. 1932), vol. 1, p. 249

   Synonyms: nightmare

3. broadlyAny oppressive thing or person; a burden.

   Again he felt the impulse of flight: but his body was a dry dead incubus that refused to obey his volition.

   August 1935, Clark Ashton Smith, Weird Tales, "The Treader of the Dust"

   2 more examples

   Ahead of us the lowering smoke-screen of Leeds and her gloomy satellites hung like an incubus over the land.

   1949 March and April, F. G. Roe, “I Saw Three Englands–2”, in Railway Magazine, page 82

   Notions of civic virtue were at that moment changing, in ways which would make of Louis's alleged vices an incubus on the back of the monarchy.

   2002, Colin Jones, The Great Nation, Penguin, published 2003, pages 132–3

4. biologyOne of various of parasitic insects, especially subfamily Aphidiinae.