BrikThulhu's head has nine tentacles and three eyes, so it's not really an octopus - it's more properly referred to as a Novapus or a Ragnoroktopus. (Or, a really fucked up demon squid as many people in the Brikverse know him to be).
Brikthulhu is the immortal king of mayhem. And It is he who makes nightmares, who sends forth his mutant spawn to sow destruction among creatures with the sentience to appreciate it. Little more than superstitions are known of him, as all who have ever seen him tear out their own eyes or are turned instantly to stone. It is possible that he is related to medusa but that is a mere speculation.
BrikThulhu is the embodied avatar of the most malevolent forces of entropy and chaos. When you spend a year and a half building a brik model of Yggdrasil, and it mysteriously falls to the ground the night before the photo shoot, smashing into a million pieces, the sound you hear is BrikThulhu laughing. You'll recognize it because it sounds a lot like your own crying.
BrikThulhu is also worshipped by the Blood Daemons.