In Western medieval legend, a succubus (plural succubi) or succuba (plural succubae) 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.
The word "succubus" comes from an alteration of the Late Latin succuba meaning "strumpet". The word itself is derived from the Latin prefix "sub-" which means "below, underneath", and the verb "cubo" which means "I lie". So a succubus is someone who lies under another person, whereas an incubus (Latin "in-" in this case stands for "on top") is someone who lies on top of another person.
Mare was also a term used to describe the sighing, suffocative panting, or an intercepted utterance, with a sense of pressure across the chest, which occurs during sleep. These symptoms were also thought to be an incubus (or succubus), an evil preternatural being, causing nightmares and/or nocturnal emissions. This phenomenon is now thought to be an experience of sleep paralysis.
The appearance of succubi varies, but in general they are depicted as alluring women with great beauty, often with demonic batlike wings, and large breasts; they also have other demonic features, such as horns and cloven feet. Occasionally they appear as an attractive woman in dreams that the victim cannot seem to get off their mind. They lure males and in some cases, the male has seemed to fall "in love" with her. Even out of the dream she will not leave his mind. She will remain there slowly draining energy from him until death by exhaustion. Other sources say the demon will steal the male's soul through the act of intercourse.