"That shocking final peril which gibbers unmentionably outside the ordered universe, where no dreams reach; that last amorphous blight of nethermost confusion which blasphemes and bubbles at the centre of all infinity . . . who gnaws hungrily in inconceivable, unlighted chambers beyond time amidst the muffled, maddening beating of vile drums and the thin, monotonous whine of accursed flutes"
- – H.P. Lovecraft, The Dream-Quest of Unknown Kadath
Azathoth, also known as the Daemon Sultan, the Nuclear Chaos, the Blind Idiot God 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, though he's too retarded to notice himself doing so (more on that below).
He is described as being both at the "center" of the universe and the "corners" of the universe, yet also existing outside of regular space and time. Despite possessing power beyond comprehension, he is an utterly mindless being, totally unaware of the cosmos around him or even of his own existence (thus the nickname "blind idiot god"). He is always surrounded by other equally mindless creatures, called Servitors of the Outer Gods by Call of Cthulhu, who play drums and flutes (very badly, we might add) while dancing around him endlessly.
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.
Despite being braindead and asleep, Azathoth can apparently have avatars like any other Outer God - likely produced subconsciously since he's too dumb as bricks to do so consciously. These Avatars range in intelligence, from just as braindead as the whole to the otherworldly intellect of the other Outer Gods - said intelligence likely has to do with whatever is sacrificed to summon them (i.e. sacrificing a human makes it sentient, while sacrificing the family dog will just net you an animalistic Avatar).
Avatars of Azathoth (seemingly) only arise in derivative works (explaining the intelligence variation since some authors forget Azathoth is actually stupid instead of metaphorically stupid), so Lovecraft purists might dislike any mention of them.
Since Azathoth is part of the Cthulhu mythos he obviously plays a role in Call of Cthulhu, though due to his nature is consigned to a background role since if he got directly involved reality would go kaput. Players will still encounter cults to Azathoth however, since here the Nuclear Chaos's sleeping self still affects reality and apparently the worship somehow influences the subconscious manipulation. Said cults are probably the most dangerous in the long term since they run the risk of poking the sleeping bear one too many times and getting all things turned to nothing.
Outer Entity Azathot from Yu-Gi-Oh