In the Cthulhu Mythos, Azathoth is the name of an Outer God and of the most powerful being and creator of the universe. Azathoth, for all purposes, is God in Lovecraft's fictional universe.
Appearance: A horrid mass far too gigantic and hideous to comprehend. Azathoth also lacks any eyes or even a mind, thus is blind and mindless.
Lore: Azathoth sits on his dark, chaotic throne at the center of the Universe and is eternally entertained by demonic flutists.
It is likely that Lovecraft based Azathoth of of the biblical God as well as the Gnostic Demiurge. As an atheist, Lovecraft would have recognised the absurdity and monstrosity of God as portrayed in the bible, i.e. an all powerful being who creates monster servants (angels) to serve and entertain him. Lovecraft was also likely attracted to the Gnostic Demiurge, the belief that the creator God was evil or at the very least, incompetent (blind idiot), hence not worthy of worship.
Powers: To powerful to comprehend
Defense Against Azathoth: Practically none, save the Elder Sign which repels him.