A Succubus was one type of horrible demon. In Mediaeval times when people didn't know any better people imagined that devils or demons would make themselves look like beautiful women to seduce men to sin. Demons of that type were called Succubi because they sucked a man's vital force from him. A beautiful succubus would drain a man of his energy and lead him to his death. She would also collect his semen and become an incubus to seduce and impregnant women.