Demons (male and female, respectively) who seek to have sexual intercourse with sleeping humans. In medieval Europe some believed that union with an incubus resulted in the birth of witches, demons, and deformed human offspring. Merlin was said to have been fathered by an incubus.
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A succubus (plural succubi) is a demon who takes the form of a beautiful woman to seduce men, especially monks, in dreams to have sexual intercourse, according to the medieval European legend. Their male counterpart is the incubus. They draw energy from the men to sustain themselves, often until the point of exhaustion or death of the victim. One such story relates to a man in the town of Coblenz, who has been bewitched by a succubus, with whom he is forced to repeatedly fornicate, whilst in the presence of his wife. The story goes on to say that "after an incredible number of such bouts, the poor man at last sinks to the floor utterly exhausted." From mythology and fantasy, Lilith and the Lilin (Jewish) and Lilitu (Sumerian) are in redactive Christian fables (folktales not part of official Christian theology), considered succubi.
According to the Malleus Maleficarum, or "Witches' Hammer", written by Heinrich Kramer (Insitoris) in 1486, Succubi would collect semen from the men they slept with, which incubi would then use to impregnate women, thus explaining how demons could apparently sire children in spite of the traditional belief that demons were incapable of reproduction through generative or gestative means. Children so begotten were supposed to be those that were born deformed, or more susceptible to supernatural influences.
In Sri Lanka and other Buddhist cultures the Succubus is named as "Mohini" , where she would wait on the road with a white dress with a baby and ask for Men's help.