Captured By the Dark Commander by Ellie Pond

Four hearts, one destiny.

As a merman commander, my responsibilities are clear: protect my people and follow the council’s orders. Despite my rank, I’ve never been good at following orders. When we discover Annabelle Portsmouth has the gene we’re looking for, I know we need her. It’s a rare trait that gives her the potential to become a mermaid. But after one look at her, I’d want her even without it. And I’m willing to take down my own men who touch her without permission.
I do what I have to do – I take her from her apartment and bring her under the sea to the domed Veiled City. A world of magic and secrets, where the females have the freedom to choose as many mates as they wish.
I understood my actions would have consequences. My government, like me, doesn’t give second chances. I did it for my nation, and for Annabelle, to let her become her true self.
But now she’s mine.
And I’ll do anything to keep this human. Anything.
And I’m keeping her.