Repenter (Players of the Game Book 1) by James McGowan

Ashe Stelfire takes what he wants and leaves the rest burning. A raider, a pyromancer, and nobody’s idea of a hero. But something is spreading through realms where magic and machine collide. A disease that thinks. That has preferences. Stopping it means becoming a man he swore he’d never be. Players of the Game, Book 1.