Citadel of Fear

Gertrude Barrows Bennett was the first major female writer of fantasy and science fiction in the United States, publishing her stories under the pseudonym Francis Stevens. Bennett wrote a number of highly acclaimed fantasies between 1917 and 1923 and has been called "the woman who invented dark fantasy". Citadel of Fear is considered Francis Stevens' masterpiece, by Lovecraft's acclaim. Two adventurers discover a lost city in the Mexican jungle. One is taken over by an evil god while the other falls in love with a woman from the ancient Mexican city of Tlapallan. Citadel of Fear was first published as a serial in Argosy Magazine in 1918-1919. It was eventually republished as a novel in 1970. It is now considered a "lost classic". (Summary by Wikipedia and Mark Nelson)