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The following is a short entry on Succubi in novels written by Terrt Pratchett.
"A female demon what comes in the night and makes mad passionate wossname", according to the parrot in Eric, a succubus on Roundworld is a demon who takes the form of a beautiful woman to seduce men (especially monks) in dreams to have sexual intercourse.
They draw energy from the men to sustain themselves, often until the point of exhaustion or death of the victim. According to the Roundworld version of the Malleus Maleficarum, or "Witches' Hammer", succubi would collect semen from the men they slept with, which incubi would then use to impregnate women. Children so begotten were supposed to be more susceptible to the influence of demons.
Eric Thursley's grandfather could never successfully conjure a succubus. Rincewind calls them "Bloody dangerous things."