Ruin by Karen Tomlinson

She is a legend…and she will set worlds ablaze to get her mate back.
Four years ago, I made a vow: I would protect what was mine. And Connor is mine. Now, he is imprisoned and being tortured. All because Hell wants me. I am the key to Satan’s freedom. And once he is free, he will devour our world.
Trapped in the realm of Faerie, I must find a way to escape the High King and return to Earth.
They tell me my mate is dead.
They tell me I’m the key to unlock Hell.
They tell me I cannot leave Faerie. They tell me a lot of things. So I smile, knowing they have forgotten; I am the stuff of legends. And they cannot contain me.
I will escape, and I will find my mate. And I will bring utter Ruin to every realm I cross until Connor is back with me—or I bring Hell to its knees.