Azathoth, also known as the Daemon Sultan, the Nuclear Chaos, and many other titles, is the big cheese of H.P. Lovecraft's Cthulhu Mythos. The first and most terrifying of the Outer Gods, powerful to the point that even the other Outer Gods would prefer he stayed asleep. Why is that you might ask? Because if he woke up, all of existence would well, cease to exist because all of reality is in fact his dream. Really. He makes the Chaos Gods look like spoiled toddlers in comparison. He also can still directly influence reality while sleeping as well (he's likely lucid dreaming, given his power would make him aware that reality is his dream).
He's also the origin of all of the Eldritch pantheon (Outer Gods and Great Old Ones included), having created the original first generation of Outer Gods by splitting off parts of himself into new beings. As a result he's Cthulhu's great grandfather, for example. And since reality is his dream, that makes humanity technically also his creations, though they're too insignificant to the Outer Gods, much less him, to be worth recognizing as such. Doesn't stop said humans who know of him from worship though.