incubus

1. (mediaeval folklore) An evil spirit supposed to oppress people while asleep, especially to have sex with women as they sleep.

2. (by extension) Any oppressive thing or person; a burden.Examples: "Again he felt the impulse of flight: but his body was a dry dead incubus that refused to obey his volition."; "Ahead of us the lowering smoke-screen of Leeds and her gloomy satellites hung like an incubus over the land."; "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."broadly

3. (entomology) One of various of parasitic insects, especially subfamily Aphidiinae.