These demons are the female verson of the Incubi. They attach themselves to a sleeping man disguised as a part of their host's body (a mole, birthmark, hair, etc.), only appearing in their true form and size after their host has fallen asleep. They send their victim horrible dreams, and during these dreams they are able to reproduce.
In Medieval times, Princess Nahemal, who roughly corosponds to the goddess Lilith, was thought to be the leader of the sucubi.