Love & Monsters by Raven Flanagan

Mina goes out for a moonlit jog when she suddenly falls through a portal into a monstrous world of red skies, demons, and creatures of unimaginable horror. And she’s the only human there.
Captured and taken to be offered as a pet to the Lord of the City, she’s imprisoned in a dungeon cell already occupied by a strangely attractive and powerful prisoner, Vadric.
Sparks fly, and fate throws Mina and Vadric together on the journey of a lifetime. While discovering pleasure that she never could have imagined, Mina must find her strength in a dangerous world.