The Citadel of Fear

The Citadel of Fear by Francis Stevens. It tells the tale of two adventurers, an Irishman and an American, who come upon the lost Aztec city of Tlapallan, in the wilds of Mexico. One is taken over by an evil god while the other falls in love with a woman from Tlapallan. Back in the states, the possessed man begins to use magic to mutate civilians.  

At the home of the Irishman's sister, a rampaging monster trashes the house one night, and that is only the beginning of one incredibly wild ride with many twists and breathtaking surprises. Nightmare creatures, battling Aztec gods and indoor swamps.

Francis Stevens - "the woman who invented dark fantasy"