Birthright

Birthright is the first book of the Sin War trilogy, written by Richard A. Knaak and published in 2006.

Description
Since the beginning of time, the angelic forces of the High Heavens and the demonic hordes of the Burning Hells have been locked in an eternal conflict for the fate of all Creation. That struggle has now spilled over into Sanctuary -- the world of men. Determined to win mankind over to their respective causes, the forces of good and evil wage a secret war for mortal souls. This is the tale of the Sin War -- the conflict that would forever change the destiny of man.

Three thousand years before the darkening of Tristram, Uldyssian, son of Diomedes, was a simple farmer from the village of Seram. Content with his quiet, idyllic life, Uldyssian is shocked as dark events begin to rapidly unfold around him. Mistakenly blamed for the grisly murders of two traveling missionaries, Uldyssian is forced to flee his homeland and set out on a perilous quest to redeem his good name. To his horror, he has begun to manifest strange new powers -- powers no mortal man has ever dreamed of. Now, Uldyssian must grapple with the energies building within him -- lest they consume the last vestiges of his humanity.

Summary
The angel Inarius was the Commander of the Heavenly Hosts. Tired of the Great Conflict between angels and demons, Inarius sought to abandon the High Heavens luring angels who thought likewise to follow him. Rogue demons who were also tired of the eternal conflict allied with Angel Inarius, and together created Sanctuary. Inarius stole from the energies of creation to build the Worldstone which kept Sanctuary cloaked and hidden from the view of the High Heavens and the Burning Hells. With this, Inarius and Lilith (Mephisto's daughter) procreated Rathma and other children, the firstborn...known as the nephalem.

Lilith betrayed Inarius and attempted to use their children against the High Heavens. Inarius, however, killed many of their children. The remaining ones became depowered when Inarius altered the Worldstone to not just keep Sanctuary hidden, but also to depower the Nephalem into mere mortals. The descendants of the firstborn became Humans as centuries passed. Lilith was not dead as Inarius thought, and returned to awaken the dormant powers of Humans by choosing a farmer with potential ... Uldyssian ul-Diomed. Through deception, Uldyssian was hunted and accused of assassination of members of the Church of the Triune and the Cathedral of Light. Uldyssian in his quest awoke others unleashing a chain reaction of hundreds of human mortals with unbelievable powers who followed him unquestionably as a leader and teacher.

Uldyssian discovered his legacy and Lilith's deception backfired ending in her apparent demise and the destruction of the Church of the Triune. The Triune taught humans about the spirits of Mefis, Dialon and Bala (standing for Mephisto, Diablo and Baal). Its leader was the Primus ... no other than Mephisto's son Lucion. The Cathedral of Light was led by the Prophet ... secretly Angel Inarius in human form. Uldyssian was caught between both forces seeking to either destroy him or possessing him. For whoever got such prize would set the balance between victory over the High Heavens or the Burning Hells.