This Hollow Heart by Nicki Chapelway

A necromancer, A ghost hunter, And the headless horseman that threatens to make them both members of the undead.

Natasya is a heretic, a necromancer, and the daughter of a crime lord. But she plays a very convincing role in making everyone believe that she’s just an innocent beauty and the daughter of a wealthy merchant. None are so fooled as her fiancé Brom the Bones, a man she is marrying to lay a claim on his inheritance: an ancient spellbook full of deadly conjuration spells.
But when Evengi, a priest of Neltruna arrives at the town of Sunder Hollow, Natasya begins wondering if she truly wants to marry Brom. Perhaps it is the draw of the forbidden or the thrill of flirting with danger, but she finds herself unable to resist this ghost hunter.
Until Brom disappears and his spellbook with him.
Natasya is convinced that Evengi and his jealousy had something to do with Brom’s disappearance, but Evengi claims it was Natasya herself who made her betrothed vanish. Each sets out to find proof of their claims. However, while they are busy pointing fingers at each other, there is something dark stirring in the mass grave of Heretic’s Rest. Something that threatens both Natasya and Evengi.

Something that lost its head and is now out for blood.