In the Middle Ages, people in Europe believed that Incubi (one Incubus, several Incubi) were male demons. There were also female demons, called succubi. They lay on sleeping people, to have sexual intercourse with them. They would also do this to make other incubi. While they had sex with their victim, they drained its energy to sustain themselves. Sometimes, having sex with an incubus may result in a child, as in the legend of Merlin. Some sources say that the incubus may be identified by its unnaturally cold penis. Religious tradition holds that repeated intercourse with such a spirit by either males or females may result in bad health, or even death.