succubi

from The Collaborative International Dictionary of English v.0.48

Succubus \Suc"cu*bus\, n.; pl. {Succubi}. [See {Succuba}.]
   1. A demon or fiend; especially, a lascivious spirit supposed
      to have sexual intercourse with the men by night; a
      succuba. Cf. {Incubus}.
      [1913 Webster]

   2. (Med.) The nightmare. See {Nightmare}, 2.
      [1913 Webster]