The Guardian of Xibalba - The Destroyer King, #1

An old friend has sent you a book he swears is true, about a young man born under a curse to become the avatar of an eldritch god, who will destroy mankind's civilization and slay its people. When cultists attempt to kidnap Jonathan Goldspear, he is forced to leave his comfortable Victorian life behind and join an expedition to last untamed land in the world. There, he will encounter eldritch horrors and betrayals, deep and bitter, yet also unexpected love and adventure, as he fights both his destiny and the ones who would twist him to their own dark ends.
This series of five books is also a story within a story, for it chronicles the descent of your old friend into madness, as the gateway between the real world and the Elder Darkness cannot be fully closed, and its eldritch beings have begun to take an interest in him. The word count of the first book is approximately 120k.