Feral is the Beast by Nisha J Tuli

The Feral King thinks he'll break me. But I will break him first.
I'm a 400-year-old witch covered in tattoos—the source of my magic—but I've been hiding them for years, trying to escape my past.
After spending centuries at the mercy of royals who thirst only for power and the desire to make up for their less impressive "assets," I refuse to be used as a tool for evil ever again.
It's any other night in my apothecary when the Feral King—a beast who eats the local villagers for sport—kidnaps me and demands my help.
When I deny him, he tries to torture me into compliance, but pain and I are long-acquainted friends, and I will never be the one who falls.
This is a high-heat immortal witch and mortal man age-gap fantasy romance with enemies to lovers and forced proximity. Perfect for fans of Sarah J. Maas, Raven Kennedy, or Jennifer L. Armentrout. Feral is the Beast is book 2 in Cursed Captors, a series of interconnected standalone stories.