The Cthulhu Mythos is a group of stories about scary, unknown creatures. These stories were created by a famous American horror writer named H. P. Lovecraft, who lived in the 1800s and 1900s. The characters in Lovecraft's stories are usually powerful or ancient gods from beyond our universe, or supernatural monsters that can cause great fear and terror in people who encounter them. Many of these creatures have unknowable names, but Cthulhu is the most famous one. He is a giant, green monster with an octopus-like head, and wings that stretch across the sky. He lives underwater and sleeps in a city called R'lyeh. People who have encountered Cthulhu say that seeing him can be so terrifying that it drives them mad. If you remember him, you will be haunted by bad dreams. Thankfully, Cthulhu usually only appears in dreams, so it's unlikely that you'll encounter him in real life!