What is the theme in Tales of the Cthulhu Mythos by H. P. Lovecraft?

The dominant theme of any Cthulhu Mythos tale is obviously horror. Indeed, the genre is defined by the horrible aspects of the story. The horror presented is distinct from violence, gore, or explicit acts of torture or barbarity. Indeed, the typical story features little if any violence and scenes of gore are more-often inferred than described. One notable exception is Howard's The Black Stone where acts of human torture and sacrifice are described in detail.