Source : Webster's Revised Unabridged Dictionary (1913)

Demogorgon \De`mo*gor"gon\, n. [First mentioned by Lutatius, or
   Lactantius Placidus, the scholiast on Statius, perh. fr. Gr.
   ? god, deity + gorgo`s fierce, terrible]
   A mysterious, terrible, and evil divinity, regarded by some
   as the author of creation, by others as a great magician who
   was supposed to command the spirits of the lower world. See
   {Gorgon}.

Source : WordNet®

Demogorgon
     n : (Greek mythology) a mysterious and terrifying deity of the
         underworld