What are the Great Old Ones and why do they make such good fictional villains?

There are a lot of fictional properties that contain lore about the Great Old ones, and all of them go back to H.P. Lovecraft's creations in his rather "unreadable" horror fiction. I say "unreadable" because there's almost no dialogue. Reading them is like reading a first hand account of events but with narrators describing things in terms that no one alive today would use. They are going to appear in the last season of Sabrina on Netflix (assumedly) in force, and it seems like a very good choice as the whole Satan storyline seems to be a bit played out. Additionally, they are in the Dungeons & Dragons fifth edition game as the power behind the warlock class, they are in movies like Hellboy and the Cloverfield franchise (loosely), they definitely inspired The Mist (a Stephen King story), and they make their rounds in online memes during political season with "Why vote for the lesser evil? Cthulhu (insert date)." I could probably name a dozen other times the Great Old Ones either directly inspire something in a story or directly contribute to it. HBO is about to add a new series called Lovecraft Country, which will (no doubt) have Great Old Ones in it. So, I'm here to ask and perhaps answer the question: what are the Great Old Ones and why do they make such good fictional villains?

The Great Old Ones are a group of unique, malignant beings of great power created by H.P. Lovecraft. They reside in various locations on Earth, and they once presided over the planet as gods and rulers. They go by strange-sounding names like Azathoth, Shub-Niggurath, and Cthulhu. In nearly all of the stories featuring Great Old Ones, there's a common theme of human insignificance and cultists. There are always cultists.

These are people who have noticed that there is actual, physical proof of one of these things existing, which makes them unique as the only godlike figures with definite presence behind them. The cultists seem to not realize that these gods of the world have no good side at all, and they stumble over themselves to get on the good side of the "true religion" as fast as they can. And then there's usually the subject of timing, which mostly has been set in the 1930's, but has found success in modern and future timelines as well. Here's a bit on that "timing" part from the tabletop roleplaying game, The Call of Cthulhu:

"When the stars were right, They could plunge from world to world through the sky; but when the stars were wrong, They could not live. But although They no longer lived, They would never really die. They all lay in stone houses in Their great city of R’lyeh, preserved by the spells of mighty Cthulhu for a glorious resurrection when the stars and the earth might once more be ready for Them. But at that time some force from outside must serve to liberate Their bodies. The spells that preserved Them intact likewise prevented Them from making an initial move, and They could only lie awake in the dark and think whilst uncounted millions of years rolled by. They knew all that was occurring in the universe, but Their mode of speech was transmitted thought. Even now They talked in Their tombs. When, after infinities of chaos, the first men came, the Great Old Ones spoke to the sensitive among them by moulding their dreams; for only thus could Their language reach the fleshly minds of mammals...

That cult would never die till the stars came right again, and the secret priests would take great Cthulhu from His tomb to revive His subjects and resume His rule of earth."

So why do they make good villains? Rather, why do people (authors and writers) mine Lovecraft's mythos for villains to insert into fantastical horror fiction? I think the answer is a bit...complicated. For one, they are completely invincible in comparison to humanity, so as far as a threat goes, it's always high-stakes (which makes for great storytelling). Second, their alien nature is so utterly bizarre that it creates madness in all who gaze upon them. This is also a great storytelling hook. Who hasn't been entranced by the rantings and ravings of a mad person in a fictional story? Finally, Lovecraft himself said something about them which gives a clue as to why they are great villains. He said that the Great Old Ones were meant to be amoral rather than malicious. This is in keeping with his belief that the universe itself was alien and uncaring, which makes them in many ways "unknowable." Whereas in the figures of Satan or in undead, we may see something familiar, I think that the horror of the Great Old Ones is increased because they are unlike anything we can imagine. The closest thing might be something we see in the Prometheus movies, but even that (I think) falls far short of the apocalyptic awfulness of the Great Old Ones.