The Necromancer's Gambit (The Gambit, #1‪)‬

Knight, the sheriff of a local magical government known as "the Gambit," is called to recover a mutilated body, tainted with magic and dumped at a popular haunt. When the corpse is identified as a close associate of the Gambit, it threatens the safety of the community he protects, and those he cares about most. As the fragile peace amongst the city's magic-wielding factions disintegrates, Knight must track down a cadre of murderers before his friends are picked off, one by one- with each death used to strengthen the spells cast against the Gambit.