H. P. Lovecraft 
Call of Cthulhu 

Hidden away, buried, in a dark underground city deep under the ocean, Cthulhu is older than the sun and the stars. Like nothing that has ever walked the earth, he is part man, part dragon and part octopus; he is a being of unimaginable cosmic proportions: beholding his form is enough to drive the sanest man into the lowest pits of hysteria and despair. Although he is near impossible to find, even for the most devout and deranged of his followers, he has the power to find you: he has the power to invade your dreams and unhinge your thoughts forevermore. (Goodreads)