The Darkest Secret (Lords of the Underworld);

Keeper of the demon of Secrets, Amun can manipulate the darkest thoughts of anyone nearby. But when the immortal warrior is chained and isolated to protect those he loves, death is his only hope of release—until he meets Haidee, a fellow prisoner whose beauty and hidden vulnerability draw him into a reckless test of his loyalty….

Haidee is a demon-assassin, raised to despise Amun's kind. Yet how can she hate the man whose touch sets her aflame? But to save him, she must give herself body and soul…and face the wrath of a powerful adversary sworn to destroy her.