Neuralger

According to the parrot, in Eric, a neuralger was the unsuccessful result of Eric Thursley's grandfather's attempts to conjure a succubus.

A neuralger is a female demon which comes to men in their dreams and has a headache. They are usually summoned by mistake, by demonologists who were expecting a succubus.

Annotations

A similar concept appears in Pratchett's (non-Discworld) drabble “Incubust”.

From neuralgia, the medical term for a painful disorder of the nerves often resulting in severe headaches.