A terrible deity, whose very name was capable of producing the most horrible effects. Hence Milton speaks of “the dreaded name of Demogorgon” (Paradise Lost, ii. 965). This tyrant king of the elves and fays lived on the Himalayas, and once in five years summoned all his subjects before him to give an account of their stewardship. Spenser (book iv. 2) says, “He dwells in the deep abyss where the three fatal sisters dwell.” (Greek daimon, demon; gorgos, terrible.)
Must I call your master to my aid, At whose dread name the trembling furies quake, Hell stands abashed, and earth's foundations shake?
Rowe: Lucan's Pharsalia, vi.
When the morn arises none are found, For cruel Demogorgon walks his round, And if he finds a fairy lag in light, He drives the wretch before, and lashes into night.
Dryden: The Flower and the Leaf, 492-5.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894