Complete Monster: Although the shared universe of H. P. Lovecraft and his contemporaries generally eschew conventional morality, many characters still go the extra mile to stand out in the bleak universe:
Nyarlathotep, the Crawling Chaos and the dreaded voice and soul of the Outer Gods, is a malignant sadist of a god with an all too human-like personality. A devious trickster by nature who enjoys playing sadistic games with mankind for its own amusement, Nyarlathotep wanders the Earth in a thousand avatars, stringing the night with the screams of those plagued by the horrid nightmares he induces wherever he walks. On record, Nyarlathotep ruins entire societies in the form of the Black Pharaoh, possesses and murders men as the Haunter in the Dark, and personally attempts to spirit Randolph Carter away into the throne room of great Azathoth itself. Nyarlathotep seduces men into worshiping his many avatars and orchestrates madness by the masses wherever he goes, differing from his fellow Outer Gods by virtue of being a wholly evil entity hindered by none of his brethren's eldritch mindsets and possessed of nothing more than a lust for reaping the chaos that defines it.
The Temple: Lieutenant-Commander Karl Heinrich, Graf von Altberg-Ehrenstein, commanding officer of the U-29 during World War I, is introduced sinking the lifeboats from a freighter he's torpedoed, making it clear he's done this many, many times before. When the supernatural begins to interfere with the crew of his ship, causing them to experience feelings of guilt and remorse over the lives they've taken, Heinrich has those affected scourged, before escalating to shooting anyone who objects or mutinies. Killing almost the whole of his crew, and aiding his executive officer in committing suicide, Heinrich dies alone on the ocean floor, a victim of his own evil as much as the supernatural.
The Case of Charles Dexter Ward: Joseph Curwen is a wicked necromancer and slave trader in the 1600s, routinely buying and murdering slaves for his dark rituals. Finally hunted down and destroyed, Curwen uses magic to ensure one of his descendants will resurrect him. Upon Charles Dexter Ward doing so, Curwen attempts to revive his old practices, including conjuring the spirits of humanity's wisest figures and torturing them for knowledge in dark rites. When Ward finally objects to the bloodshed Curwen propagates, Curwen murders him and takes his place, thrown in an asylum when he can't properly pass as his descendant due to his antiquated mindset. It is further revealed Curwen is allied to a horror from beyond, where his plans could lead to the utter annihilation of all life in creation.
The Thing on the Doorstep: Ephraim Waite, the father of Asenath Waite, was a man affiliated with various dark covens and powers in life who sought immortality regardless of the suffering he inflicted upon others. Consulting dark tomes for a way to live forever, Ephraim finally found a way to expand his own life by body-swapping with his own daughter and damning her to insanity and slow death within his own old body as he took hers. Seeking a male body with strong intellect and weak will to him to weaken and possess, Ephraim slowly seduced Edward Pickman Derby while slowly driving him further and further into madness. When Derby killed Asenath in a desperate bid to stop Ephraim, Ephraim ultimately swapped bodies with him and condemned him to horrific undeath within the rotting corpse of Asenath. Unstopped, Ephraim's dark practices would destroy the peace and comfort of the world as he unleashed untold horror on humanity, jumping from body to body and damning countless more innocents to death for all time.
Y'golonac, the Defiler, is the god of depravity and a Great Old One reviled even by the priests of Cthulhu for its unspeakable perversions. Gathering cults of those with carnal hearts, Y'golonac has his followers engage in whatever grotesque fantasies they can imagine in tribute to it, hoping to eventually break out of its walled prison and walk free among men before wiping out all humanity with the other Great Old Ones. A sadist fully able to understand and manipulate humans, Y'golonac possesses people at its own merriment and gruesomely devours all those who do not pledge themselves to serving it. Debuting In Ramsey Campbell's 1969 Cold Print, Y'golonac possesses a bookseller in lower Brinchester and drives a man to insanity to draw in more victims for it to subvert or eatchildren among them—and ultimately closes the story by devouring the protagonist himself.
Until you re-read it, and realize that it undoubtedly IS a parody of the Crucifixion...
Fanon: A lot of the "mythos" could be considered this, since so much was built up by later authors, rather than Lovecraft and his contemporaries. Of particular note is the belief that "Dagon" is a Lovecraftian God and a servant of Cthulhu. Pretty much any reference Lovecraft makes to "Dagon" in his own stories could be read as in-universe mythological allusions, out-of-universe mythological allusions, or a code name used by Cthulhu worshippers to avoid attracting attention.
Fanon Discontinuity: While there are plenty of canon disputes within the fandom, Bloch's "Shadow From the Steeple" is commonly held in low regard.
Lovecraft described Azathoth as a monstrous indescribable thing which resides in the center of the universe, often described as "gnawing" and "chaotic". Scientists now believe that the center of our galaxy (and, by extension, other galaxies), is a supermassive black hole. Perhaps Lovecraft was on to something...
In the 1931 short story The Lair of the Star Spawn by August Derleth and Mark Schorer, the characters manage to stop the Great Old Ones Lloigor and Zhar with the aid of the Star Warriors from Orion, described as monstrous-size glowing beings that "shot great beams of annihilation and death". In 1966, Tsuburaya Productions created Ultra Series, a toku series about heroic aliens, also known as giants of light, with many abilities, including shooting powerful beams from their arms. And by sheer coincidence, their homeworld is located in Nebula M78 in the Orion Constellation. Doubly hilarious after their 1996 entry Ultraman Tiga, which featured Ghatanothoa as the Big Bad, complete with Lloigor (or Zoiger in this case) servants and an appearance from R'lyeh.
Inferred Holocaust: The bodies most commonly associated with the Yithians? Those belonged to beings native to Earth before the Yithians used their mind-swapping powers to leave their own dying world for ours...
Hastur originally came from Ambrose Bierce's Haita the Shepherd. Chambers used the name in The King in Yellow. In spite of Hastur's prominence in the works of later Mythos authors, H. P. Lovecraft only used it once as a casual name-drop in "The Whisperer in Darkness".
The term Fire Vampires were first used for Fthaggua's servitors. Latter, Call of Cthulhu used the term to describe Cthugha's Flame creatures, and became the depiction that most associate with.
"Seinfeld" Is Unfunny: Similar to Slender Man in present day, some consider the character of Cthulhu to be so overused and well-known that he's become a cliché of the Cosmic Horror Story genre. Fortunately, there's plenty of other monsters and villains in the Mythos for writers to use to avert this.
One could consider the interbreeding between humans and the Deep Ones to be this since it's basically people having sex with giant fish.
Too Dumb to Live: The protagonist of "Fane of the Black Pharaoh" by Robert Bloch. Being led down by a cultist, the archeologist sees centuries of prophetic events, notably of visitors being lead down the Fane and soon killed. Guess what happens next?
Word of Dante: Several common aspects of the Cthulhu Mythos (such as the good/evil dichotomy and the Necronomicon as a powerful Brown Note) come from Lovecraft's friend, August Derleth, rather than Lovecraft himself.