Nyarlathotep.
Nyarlathotep is an ancient cosmic being that is said to be far older than Cthulhu himself. This is due to the fact that Nyarlathotep is a member of the Outer Gods, a race of Eldritch monstrous creatures that predate the current universe and formed amidst the chaos that wandered the primordial nothingness. While most, if not many, of the Outer Gods are inactive and incapable of human understanding, Nyarlathotep differs from them. Nyarlathotep is even seen as the Devil in Lovecraftian literature as Satan is in Christian literature.
Unlike the majority of his race, Nyarlathotep is active and is tasked with wreaking havoc across the cosmos, and most notably Earth in particular. His goal is to sow despair and woe wherever he treads, thereby spreading the chaos that he was born from and to engulf the light of God in his darkness. While he claims to be a simple messenger, his actions are incredibly destructive and heinous to be looked upon as one through human perception.
Nyarlathotep is also said to be the most "human" of the Outer Gods. This is due to him assuming many human forms over the centuries he has been on Earth. This also aids him in throwing society in shambles, turning many people against each other, inciting wars and battles, revolutions, and so forth. He is known to perform this through the use of corrupt propaganda, deception, and manipulation. Because of this, Nyarlathotep is considered to be the most 'human' of the Outer Gods due to his ability to walk freely among the mortals, thus, it makes him the most terrible of the Old Ones.
Nyarlathotep enacts the will of Chaos, and is its messenger, heart and soul; he is also a servant of the other Outer Gods, whose wishes he immediately fulfills and the one member of the who deals on a personal level with human beings in general. Unlike the other Outer Gods, causing madness is more important and enjoyable than death and destruction to Nyarlathotep. He is described as the very soul of "gigantic, tenebrous, ultimate gods – the blind, voiceless, mindless gargoyles". As such, his evil is absolute, for he is not only the servant of these creatures, but their very consciousness.
Nyarlathotep has also assumed the forms of other creatures and beings, and thus causing many cultures around the world to worship and revere this particular form as a separate entity or deity. In the process, it taints and damns the religion of God. But like his race, his true form is unspeakably dreadful, and would render any being in close proximity or even from afar into a pile of blob-like nothing. This shows that Nyarlathotep must take a form that is compatible with the laws of our reality.
While he does possess numerous followers, Nyarlathotep is believed to have even spawned children known as the Million Favored Ones. Their number is obviously implied in the epithet. His children are said to be as depraved and as manipulative as he is, to the point where they would even have cults of their own and gather a collective number of people to either join them or perform a sacrifice of sorts for their benefit or, mostly, their father. But in the end, the Million Favored Ones would always follow the orders of Nyarlathotep without question as he would use them to further his agendas.