Book 4. Fire-Breathing Serpent, Vampire World and Black Fox - Demon Sword. Heart of God, #4

Medieval Times. Territory of Venezuela. Soon the Commander, Professor, Rene and a detachment of local residents go to the Golden City in order to destroy Evil. They decide to take advantage of the sacred labyrinths of Xibalba (the god of the underworld) and attract a heathen priest as a guide. The heathen priest was well versed in the endless mountain tunnels, as he made sacrifices to the sacred snake Chimalcan.
Events unfold in the most unexpected way. The sacred snake kills the heathen priest, the native people are frightened and refuse to follow on. Further, only the commander, Rene, the Professor and his wives continue on their way. They fall into a trap and realize that they will die of thirst and hunger. Rene decides to resort to a last resort: move to the Gray World. The vampires greet him warily, but he shows them Vanessa's necklace.
Vanessa hospitably welcomes the 'guests', she remembers the service rendered to her by Rene. A mysterious stranger appears. The stranger hands Hidalgo the sword of Gaborim, with which he must slay the Black Fox. The stranger admits that the Black Fox is not a person, but something that arrived on earth from the infinity of the universe. This creature is a danger to both God and the Devil...