The thirteenth skull - Richard Yancey

There are some things deep within the belly of the earth that should never be disturbed. Which is exactly why Alfred wants to stay at home. When a vision foretells a cataclysmic battle between the "Sons of Light" and the "Sons of Darkness", Alfred is called into action. Whoever finds the fabled 13th skull of Merlin, will have in his hands the power to usher in a new Dark Age … and the Apocalypse. And so the race is on to find the skull, which rests at the bottom of an abyss called Krubera - a place so terrifying it nearly drove Alfred's mentor, Op-Nine, insane…