According to 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.
In some beliefs the succubi would morph into the Incubus with its newly collected semen ready to impregnate their victims. This was to account for the fact that demons could not reproduce naturally, however the incubus could impregnate women.
From the 16th century, the carving of a succubus on the outside of an inn indicated that the establishment also operated as a brothel.
From Wikipedia:
In medieval legend, a 'succubus' (plural succubi; from Latin succubare, "to lie under") is a female demon which comes to men, especially monks, in their dreams to seduce them and have sexual intercourse with them, drawing energy from the men to sustain themselves, often until the point of exhaustion or death. This legend was an explanation for the phenomena of wet dreams and sleep paralysis. Lilith and the Lilin ( Jewish), Belili ( Sumerian) and Rusalka ( Slavic) were succubi.
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From www.hypnosisinmedia.com.
Incubus and Succubus: In medieval lore, a succubus was a female spirit that would lay with men to steal their seed: it was given as an explanation of night arrousal and wet dreams. It was also paired with the incubus, the male form, sometimes transforming between the two forms to pass along the stolen seed to unknowing women. In later stories, especially in contemporary horror writing, both would be given hypnotic powers that would give them the ability to command and compell members of the opposite sex.
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From occultopedia.com
A lewd female demon or goblin which takes on the illusory appearance of a female human being and seeks sexual intercourse with men, usually while they are asleep.
The princess of all the succubi (plural) is Nahemah. Its male counterpart is called Incubus. A semi-human offspring is called Cambion.
According to the view of most medieval theologians, incubi outnumbered succubi by nine to one, but the ladies made up in menace for what they lacked in numbers by being alluring and persuasive, using their considerable charms to seduce men and lead them to eternal damnation.
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From Legends of the Succubus
The succubus is a demon from legend that supposedly preys on mortal men while he sleeps; a sexual vampire of sorts. The actual name has its origins from late Latin- succuba meaning prostitute, which in turn comes from medieval Latin sub cubaire meaning 'that which lies beneath'. The male version is the incubus (from Latin- 'that which lies above'). There are some sources who claim that the succubus and the incubus are one and the same creature who can change form at will to prey on mortals.
In medieval times, the succubus was seen as a fearsome creature who killed her victims by drinking their breath. This is interesting in that, at the time, the breath was seen as a part of the person's spirit, and in doing so, the succubus was thought to be stealing the victim's soul. Later, the habits of the succubus were deemed to of a more sexual than vampiric nature, and this notion probably arose from the change in social climate that saw sexual deviancy as a mortal sin, and so, those who committed such a sin against God, were deserving of their fate in some way.
