Where it began
Looking back to what August Derleth would later call the Cthulhu Myhtos, we can find seven stories which acted as a base to all future books, movies or games. Most notable one is written by H.P Lovecraft - The Call of Cthulhu released in 1928, which was not regarded by the author as something special. It introduced many themes and elements shared with other works.
Lovecraftian horror
Known also under the name of 'Cosmic horror', is a subgenre of horror fiction, it encomposes the horror or uncertainity of the unknowable and uncomprehensible over the descriptions of gore-y elements or shocking the readers. One of the themes reapeating throughout many works is the forbidden knowledge which might lead to madness as well as fanatic cults worshipping cosmic entities or other powerful monstrosities.
Popular Culture
Influence of the Lovecraftian creations can be found almost everywhere, from other books through music, movies and comics on tabletop and video games ending. Phrases such as Cthulhu, Necronomicon, Arkham or the city of R'lyeh should be recognizable by some. Somre more recognizable movie titles include "Supernatural", "Lovecraft Country" or games such as "Bloodborne" or "The Sinking City".