The succubus visits men in the night while they sleep, often she will have sex with the men. Other times, she will visit them in their dreams only to obtain the man’s sperm. Once she has the sperm, she takes it to the incubus who in turn takes it to a mortal woman, and impregnates her with an average child. There is no real reason known for this (Roussaeu). Occasionally, the succubus will become lesbian, and prey on women. In the event the succubus chooses a woman partner, the succubus will visit the woman in the night, and hypnotize her. Once the woman is hypnotized the succubus will have the woman perform oral sex onto the succubus or vice versa (McAurthur).
Links for further research:
This website provides in depth information on the Incubus, Succubus, and Cambion.
The Issachar Ministry
This website tells how to gain deliverance from the incubus and succubus, and a Christian view of what this entity is may be.
My Ghost Stories
This website allows people to share their personal stories of attack with incubus and succubus.