Demon Core

Shattered by tragedy, consumed with rage, a young man surrenders his soul for the power to destroy humanity in the first novel of this dark fantasy series.
After the death of his beloved mother, Swain is left at the mercy of his drunken father. His only respite from all the beatings and abuse lies in the words he scribbles down, seeking beauty in an ugly world through poetry. He has no friends until one day he meets a girl near his mother's grave. A restless spirit who calls herself Goose, over the years she encourages Swain's creativity, eventually helping him tap into a magical ability he never knew he possessed: a dark energy entwined with his despair, fueled by his anger, and controlled by his poetic verse.
But this revelation is swiftly followed by a cruel betrayal that devastates Swain. Unleashing his power, he transforms into the Demon King and spreads a plague across the land, infecting every living creature in his path. He knows there's beauty to be found among the base and selfish people he encounters-the graceful dancer, the perfectionist painter, the master chef-and he twists their artistic desires to serve him. Hollowed out and reborn as undead entities, they join the corrupted army of inhuman beings bent on conquering those the Demon King believes allowed the world to become a horrid ruin.
Ruhr, the half-elf river-sorceress adventurer, is in the city when it's besieged by demonic forces. The invasion is her opportunity to earn royal wealth and favor. But while her water magic purifies the monstrous legion, the Demon King marches on to remake the world according to his warped vision of beauty . . .