Rebel Witch by Kristen Ciccarelli

The Crimson Moth, book 2

hardback 

A witch slayer... Not content with merely deceiving Gideon, Rune has now turned traitor, joining forces with Cressida, the very witch who tormented Gideon. Gideon refuses to let the Republic succumb to witch rule and revert to past horrors. To safeguard his hard-won world, all witches must perish-especially Rune Winters.