Red Cloak Moon by Fiona Lawless

When he’s forced to play bait after a series of grim murders, a naive omega must decide whether his fated mate is a philanthropic outlaw or a psychopathic killer.
Thierry Toussaint has no choice but to don the crisp white dress and red cape of the Sisters of the Merciful Moon and act as bait to catch a killer. As a male omega, he’s considered a freak—a mistake of nature—and therefore expendable. He’s dispatched to deliver food and aid to an old woman deep in the shelter of Bois des Voleurs forest, where picturesque, mostly benign outlaws, and a homicidal maniac, await.
Yves, the bandit of Thieve’s Woods, has enough on his mind without necromancy giving him and his pack a bad name. He’s sworn to serve the poorest citizens of Rhielôme as well as the red-cloaked Sisters of the Merciful Moon who act as counselors, nurses, and midwives to families displaced by Rhielôme’s impossibly high taxes.
When Yves saves Thierry from becoming the third victim of the “Red Cloak Killer,” they discover a fated love so powerful nothing can tear them apart.
In the sun-drenched kingdom of Rhielôme, appearances are more important than reality. There are laws on the books protecting all omega citizens against forced marriage and trafficking, but there are those who ignore them, and justice often comes at a high price. In a country that sees him as less than nothing, Thierry must risk everything—even the freedom of his fated mate—to catch a monster.