Giger heard about the CTHULHU-myth and the Necronomicon for the first time when a writer from Zurichse schrijver, Robert B. Fischer, published a magazine called the Cthulhu-News. Giger illustrated the stories of the second edition. These drawings can be found in Giger his Biomechanics.
Necronomicon is a myth about the great deities with names like CTHULHU en YOGSOTHOTH, who doze in the dephts of the earth and the see, waiting for the perfect moment to take over the world power. This as discribed in the Necronomicon of Abdul Alhazred. The deities are reflected as horrible monsters like in the stories about this subject written by J.P. Lovecraft.
In the age of Elizabeth I her astrologer John Dee and his assistant Edward Kelly went to the palace of king Rudolph in Prague to try to change led into gold. During their trip Kelly received information from the original Necronomicon from an angel in the language of the angel. Pictures of horrible deities weren't found. Only fragments have been remained form the original Necronimicon (Al Azif), which are kept in the British Museum in Londen.
When Giger was looking for a suitable title for his ominous paintings Sergius Golowin, a famous Swiss expert on myths, suggested to use Necronomicon.
In the meantime many different Necronomicon appeared but none with illustrations of alien horror figures like the ones in Giger his Necronomicon.