Rebel in the Deep by Katee Robert

The rebellion’s fight turns into a battle of the heart in this pulse-pounding conclusion to the Crimson Sails trilogy by the bestselling author Katee Robert.

Nox has been steadfastly working for the rebellion for years. They don’t ask for much in return, except for one crucial condition: their ex, the noble Bastian, stays as far away from them as possible. To say things ended poorly between them is an understatement . . . and it’s the only relationship Nox has never quite recovered from.
But now the rules have changed. Siobhan, the rebel leader, has emerged from hiding to tell Nox that Bastian’s been taken captive and the secrets he holds tight are in danger of being revealed. The fate of the entire rebellion now rests on Nox and Siobhan’s ability to rescue Bastian from a Cŵn Annwn ship.
Saving Bastian is only the start of their hardships, as the trio is tracked by ferocious pirates across Threshold. And Nox’s complicated relationships and entanglements with Bastian and Siobhan put not only their life at risk but their heart on the line.