Bravery's Sin. Trinity Masters: Masters Admiralty, #5

An unbearable loss. A man willing to kill for her. A future they'll never have. Brilliant, beautiful...and scarred, Nyx Kata is living on borrowed time. In a race to find the villain threatening to destroy the Masters Admiralty, Nyx is forced to face the terrifying secrets of her past. Secrets...and a man...she can no longer run from. Grigoris Violaris is a knight, who has sworn to protect Nyx at all costs.
He failed her once, a mistake he won't make again. There's no denying their chemistry or the deeper feelings they have for one another, but in the world's most powerful secret society, arranged menage marriages aren't created because of love. The clock is ticking as Nyx and Grigoris search for a madman. Because the threat is no longer just against the secret society. The mastermind has set his sights on them.