Perhaps the second most famous eldritch abomination in Lovecraft’s pantheon, Nyarlathotep is something of an oddity among oddities. If entities like Azathoth and Yog Sothoth are gods, then Nyarlathotep is sort of a dark angel, working as their messenger. Nyarlathotep appears in many Lovecraft stories and is mentioned in others, and, of all the major players in Lovecraft’s mythology, he has the most human-ish personality. While Azathoth, Yog Sothoth, and the other various “gods” of the mythos seem distant and removed from humanity, Nyarlathotep actively works amongst us as a cruel trickster and devil-like figure, manipulating mortals with apparent glee. When Yog Sothoth or Cthulhu destroy you, it’s basically an accident (albeit one he could care less about). When Nyarlathotep destroys you, it was probably planned.
I think Lovecraft created Nyarlathotep to sort of have his cake and eat it too. While monsters like Cthulhu are terrifying because of their incomprehensibility, stories need plots that can be comprehended by the audience to, well, appeal to an audience, which is pretty important if you want to live off your writing. So Nyarlathotep presents a sort of middle ground, acting as a somewhat flawed translator of the untranslatable. He literally puts on a human face (and a black one more often than not because, well, Lovecraft was a racist) and pretends to have simple motives, tricking human beings into sabotaging their own species by forming cults to bring the various cosmic monsters back to life and power.
However, the key aspect of Nyarlathotep is that his human persona is a façade. While he acts cruel and devilish, his goals aren’t as simple as destroying humanity – like all of Lovecraft’s cosmic beings, he ultimately wants to facilitate some grand, abstract design of the universe that just happens to be catastrophic for human civilization. The goals Nyarlathotep works for are no more inherently malevolent than, say, the eventual heat death of the universe – the fact that we would be destroyed by them is a mostly irrelevant detail as far as the laws of physics are concerned.
Nyarlathotep shows up in a lot of stories outside of Lovecraft’s canon, even being used as a pseudonym for Stephen King’s multi-dimensional agent of evil, Randall Flagg. The semi-comprehensibility of his motives and methods may be the key to his popularity, as it’s a lot easier to write Nyarlathotep’s standard characterization than it is to write inscrutable beasties like Cthulhu. Yet sometimes he slips a bit too far, becoming JUST a squid-shaped Mephistopheles when, like all of Lovecraft’s beasties, he should be something just a bit more peculiar than a simple tempter.