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6.
High priest of the great "Old Ones," alien super-beings who came to earth many millions of years ago during the Paleozoic era that were eventually worshiped as gods by the johnny come lately human race.

His name is unpronounceable by human mouths.

A rough description of Cthulhu goes as:

A pulpy, tentacled head surmounted a grotesque and scaly body with rudimentary wings ... It represented a monster of vaguely anthropoid outline, but with an octopus-like head whose face was a mass of feelers, a scaly, rubbery-looking body, prodigious claws on hind and fore feet, and long, narrow wings behind. This thing, which seemed instinct with a fearsome and unnatural malignancy, was of a somewhat bloated corpulence ...
In his house at R'lyeh dead Cthulhu lies dreaming.
by Miskatonic Jack 2 June 24, 2010
 
1.
A character in Lovecraft's tale "The Call of Cthulhu". Cthulhu is a monstrous entity who lies "dead but dreaming" in the city of R'lyeh, a place of non-Euclidean madness presently (and mercifully) sunken below the depths of the Pacific Ocean. Cthulhu appears in various monstrous and demonic forms in early myths of the human race. Racial memory preserves Him as humanity's most basic nightmare. Cthulhu is the high priest of the Great Old Ones, unnatural alien beings who ruled the Earth before humanity formed, worshipped as gods by some misguided people. It is said that They will return, causing worldwide insanity and mindless violence before finally displacing humanity forever.
"That bitch had some Cthulhu eyes."
by ephidryn December 02, 2003
 
2.
He who lies dead but dreaming. He who lies waiting under the waters, ever seeking entrance into the world of the living, so The Ancient Ones may rule the Earth once more.
"That is not dead which can eternal lie. And with strange aeons even death may die."-H.P. Lovecraft
by E April 05, 2005
 
3.
Cthulhu (commonly preceeded by "Great" or Old" is a monstrous being belonging to a group of entities known simply as the "Old Ones" which ruled over the Earth and allegedly warred with the Elder Things before the time of man. Great Cthulhu currently remains imprisoned in the great cyclopean, sunken city of R'lyeh (assumed to be near Valparaiso) in a death-like state awaiting the day the stars are right, when he will resume his glorious rule of earth in an orgy of destruction.

While psychicly sensitive humans have been contacted by Cthulhu through telepathy (which is assumed to be the "language" of the Old Ones), the only reported sighting by a human in recorded history occured on March 23, 1925. This was twenty-two days after R'lyeh rose above the water, only to sink once more shortly after when the merciful stars' shifting position caused Cthulhu to resume his death (the conditions required for R'lyeh to rise are also required for the Old Ones to live). This was reported by Gusaf Johansen, the only remaining crew member of the Emma. He later died under mysterious circumstances in his hometown of Oslo.

Cthulhu is described as resembling a green humanoid with draconian wings, and a head resembling an octopus (though it is believed Cthulhu is not composed of matter. While it is a common misconception that Cthulhu is evil, it is worth noting that he/it appears to be amoral, viewing humans as pests (akin to human views and actions with insects).
Ph'nglui mglw'nafh Cthulhu R'lyeh wagn'nagl fhtagn
by wwwwoooooooo! August 07, 2006
 
4.
A entity for H.P. Lovecraft's short story "The Call of Cthulhu" who also appears in several later stories, though CC is the only one that focuses on him. Cthulhu is a extraterrestrial being referred to as "Great Old Ones" and is generally associated with madness. It is said to have "yielded simultaneous pictures of an octopus, a dragon, and a human caricature.... A pulpy, tentacled head surmounted a grotesque scaly body with rudimentary wings." While no specifics of it's height have been given, in the Climax of Call of Cthulhu, when some sailors stumble upon the formerly-sunken city of R'lyeh, the city in which Cthulhu is said to be sealed, they accidentally release the monster, and one said it was "A mountain that walks or stumbles", implying it's enormous height.

For some reason, Cthulhu along with the other Great Old Ones have been sealed away, and is said that Cthulhu one day will awaken and release the Great Old Ones and they will resume the rule of Earth, and there will be a "Holocaust of freedom and ecstasy". the Great Old Ones telepathically talked with the earliest humans, and started the infamous Cult of Cthulhu.

many real life cults have been started and can be found on the internet. I'm not sure which scares me more, the Cthulhu cults, or the other religions.
In his house at R'lyeh dead Cthulhu waits dreaming

I had a nightmare about Cthulhu last night
by Ttam2045 February 19, 2010
 
5.
A fictional character created by H.P Lovecraft a horror writer in 1919. Cthulhu is described as " A thing too horrible to describe." Although it is often though of having an octopus-like head, with bat-like wings, and being the size of a mountain. The very sight of Chthulhu can bring a grown man on his legs, and cry like a baby.
There are nut jobs out there, REALLY OUT THERE that worship it.

I find the concept of Cthulhu, and the cults....a little freaky.
I was bored, and googled Cthulhu, and am now insane YAY!

I found a Cthulhu based board game...<- this is really true.

Cloverfield was mistaken for a Cthulhu movie.
by !INSANE IN THE MEMBRANE¡ January 23, 2009
 
7.
The Great Old One, or High preist of the Great Old Ones depending on the story you read.
Less a god than a monstrous horror resembling an octopus, man, and dragon cojoined from a creation wholy unlike birth.
Best pronounced sucking water into your lungs whilst beining dragged down by a tentical.
Anyone shewn Cthulhu, sometimes even in representations fearfully crafted by his cults, cannot survive sane.
"Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" In his city, R'lyeh, Great Cthulhu waits, dreaming.
by Cyberserker July 10, 2008