Bound

Sweet witch Ruby Waterstone would do anything to protect her town—even open a portal and offer herself to a Skullstalker, a benevolent monster who lures lost travelers into his shadowy void to be devoured.
But the void she steps into is shockingly beautiful despite its darkness. The Skullstalker is even more so. Ruby finds herself reluctantly charmed—even after the mysterious Skullstalker reveals what she must do to protect her town.

Slate is too busy sleeping away the centuries to concern himself with mortal affairs. That is, until a stray witch appears in his void, begging him to fix the failing ward protecting her town. In return, she will give him whatever he wants.

Unbeknownst to her, the ward can only be replenished by a mating ritual. And Skullstalkers can't mate with mortals without practice. Thorough, repeated practice. But it's not as if he's confining her to his nest. There's a sinister reason the ward is failing, and they must venture deep into his void to find it.

Both are surprised to discover how much Ruby thrives in his dark realm. As they get closer to fulfilling the ritual, Slate starts to realize what he wants from her when all this is over:

Her heart.Contains mature themes.