The Victorian mind was assaulted by the subliminal sensuality of the vampires in “Dracula” but in the ancient cultures we find 2 demonic entities who weren`t as subtle about the reason why they made their nocturnal visits.
Incubus and Succubus [by their latin names] were body-less demons that would materialize during the night to sexually assault the sleeping, to torture and sometimes kill them.
The nightmares were associated with anxiety and sexual repression. It was believed that in some cases it was nothing more what sexual fantasies taken to the extreme.
There creatures of the night would use their powers to paralyze the victims and sustain sexual relations with them against their will. Often those that resisted were killed.
Some claim that this demon didn`t have a sex and that it would change the form after each victim thus taking either the form of a male or a female.
It is believed that the woman [Succubus] was a demon essence that would take the form an extremely beautiful woman and seduce a wealthy and influential man and bare him a child. The baby would be a powerful demon but his human form could be sustained permanently.
Once the succubus gave birth she would entrust the demon-child to the father and it would be raised into the mansion as an orphan until his coming of age. At that point he would kill his father and take his place in the community.
In “Discoverie of Witchcraft” by Reginald Scot  these demons are associated with witches and the case of an incubus that feed on a young aristocratic maiden [the case was based on the testimony of the servant].
But not many dared to tell the story of such encounters because while the witch hunt was in progress confessing to having sexual affairs with a demon or with a devil brought a sure conviction.
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† Vampires in Ancient Cultures