InPrometheus Unbound , Demogorgon is a spirit from the underworld who challenges and ultimately dethrones his father, Jupiter. In literature, Demogorgon is traditionally presented as a diabolical character. Yet Shelley, true to his radical political beliefs, makes Demogorgon a force for good, a force that brings love and harmony...
to the cosmos.
Demogorgon's usurpation of his father is an allegory of revolutionary political change, the kind which Shelley enthusiastically endorsed. Shelley portrays Jupiter as a vengeful, sadistic tyrant, and it's not hard to see in this portrayal a reflection of Europe's ruling classes. Jupiter wants to keep humankind in ignorance and fear, the better to overawe and control them. This is precisely what Shelley accused the ruling classes of doing to the common people.
Etymologically, Demogorgon derives part of its name from the Greek word demos, meaning "people." (This root is also where we get the word "democracy" from). In that sense, Demogorgon symbolizes the common folk, the poor, oppressed masses, rising up from the depths of darkness and ignorance to overthrow the tyrannical power that has kept them in subjection for so long.