“The time would be easy to know, for then mankind would have become as the Great Old Ones; free and wild and beyond good and evil, with laws and morals thrown aside and all men shouting and killing and revelling in joy. Then the liberated Old Ones would teach them new ways to shout and kill and revel and enjoy themselves, and all the earth would flame with a holocaust of ecstasy and freedom.”
I don’t think that’s literal. In the Cthulhu mythos, the Old Ones ruled the earth eons before humans and never destroyed it. I think it means the flame of insanity.