"The Call of Ktulu" was Metallica's second instrumental song, following the first instrumental "(Anesthesia) Pulling Teeth" from Kill 'Em All. The song working title was originally "When Hell Freezes Over".
The idea of the song "The Call of Ktulu" is based upon H.P. Lovecraft's book The Shadow over Innsmouth which was first introduced to the rest of the band by Cliff Burton. The song's name was taken from one of H.P. Lovecraft's main stories featuring Cthulhu, The Call of Cthulhu, which was written in 1928 for the magazine Weird Tales. The name "Ktulu" is originally written "Cthulhu" by H.P. Lovecraft.

Cthulhu is a fictional cosmic entity created by horror author H. P. Lovecraft in 1926. The first appearance of the entity was in the short story "The Call of Cthulhu" published in Weird Tales in 1928.
Cthulhu is one of the central Great Old Ones of the Lovecraft Mythos. It is often cited for the extreme descriptions given of its hideous appearance, its gargantuan size, and the abject terror that it evokes. Cthulhu is often referred to in science fiction and fantasy circles as a tongue-in-cheek shorthand for extreme horror or evil.
After its first appearance in "The Call of Cthulhu", Cthulhu makes a few minor appearances in other Lovecraft stories.[1] August Derleth, a correspondent of Lovecraft's, used the creature's name to identify the system of lore employed by Lovecraft and his literary successors, the Cthulhu Mythos.

Physicality and origins

While the origin of Cthulhu is not definitively established, it is suggested that it is the planet Vhoorl, with his advent somehow connected with supernovae: "I learned whence Cthulhu first came, and why half the great temporary stars of history had flared forth." It is also suggested in both At the Mountains of Madness and “The Whisperer in Darkness” that Cthulhu is made up of some unknown and foreign matter.
The most detailed descriptions of Cthulhu appear in the short story "The Call of Cthulhu", and are based on the statues of the creature. One, constructed by an artist after a series of baleful dreams, 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."[8] Another, recovered by police from a raid on a murderous cult, "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."[9]
When the creature finally appears, the story says that the "thing cannot be described", but it is called "the green, sticky spawn of the stars", with "flabby claws" and an "awful squid-head with writhing feelers". The phrase "a mountain walked or stumbled" gives a sense of the creature's scale.

Cult of Cthulhu

Cthulhu is depicted as having a worldwide doomsday cult centered in Arabia, with followers in regions as far-flung as Greenland and Louisiana. There are leaders of the cult "in the mountains of China" who are said to be immortal. Cthulhu is described by some of these cultists as the "great priest" of "the Great Old Ones who lived ages before there were any men, and who came to the young world out of the sky."
The cult is noted for chanting the phrase "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn", which translates as "In his house at R'lyeh, dead Cthulhu waits dreaming."[13] This is often shortened to "Cthulhu fhtagn", which might possibly mean "Cthulhu waits", "Cthulhu dreams", or "Cthulhu waits dreaming."
One cultist, known as Old Castro, provides the most elaborate information given in Lovecraft's fiction about Cthulhu. The Great Old Ones, according to Castro, had come from the stars to rule the world in ages past.
They were not composed altogether of flesh and blood. They had shape...but that shape was not made of matter. 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.
Castro points to a "much-discussed couplet" from Abdul Alhazred's Necronomicon:
That is not dead which can eternal lie.
And with strange aeons even death may die.
Castro explains the role of the Cthulhu Cult, stating that when the stars and the earth "might once more be ready" for the Great Old Ones, "some force from outside must serve to liberate their bodies. The spells that preserved Them intact likewise prevented them from making an initial move." At the proper time,
the secret priests would take great Cthulhu from his tomb to revive His subjects and resume his rule of earth....Then mankind would have become as the Great Old Ones; free and wild and beyond good and evil, with laws and morals thrown aside and all men shouting and killing and revelling in joy. Then the liberated Old Ones would teach them new ways to shout and kill and revel and enjoy themselves, and all the earth would flame with a holocaust of ecstasy and freedom.
The character goes on to report that the Great Old Ones are telepathic and "knew all that was occurring in the universe". They were able to communicate with the first humans by "moulding their dreams", thus establishing the Cthulhu Cult, but after R'lyeh had sunk beneath the waves, "the deep waters, full of the one primal mystery through which not even thought can pass, had cut off the spectral intercourse.

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Source: Wikipedia