Age of Conan Hyborian Adventures: Heretic of Stygia Series by J. Steven York

Anok Wati, a young warrior of the streets, strikes a pact with an ancient and forbidden god who gives him a seemingly impossible task. To unravel the mysteries of the past and avenge his father's death, Anok must join the sinister cult of the snake-god Set--and destroy them from within.

Seeking his father's murderer, the warrior Anok has joined the Cult of Set. Tainted by dark sorcery, he begins a perilous journey across the desert to a city of outlaw sorcerers in order to control his magic before it consumes his soul.

Second of three epic trilogies set in the legendary world of Conan the Barbarian.  Cursed with an unspeakable power, Anok Wati, the heretic of Set, has unknowingly unleashed a hideous evil. And unless he can find the hero within himself, and destroy a god given flesh, all of Hyboria will be enslaved.