Demon's Destiny. Guardians, #4

Renowned for his arrogant demeanor and notorious playboy lifestyle, Baal revels in his freedom to defy conventions. As the most formidable demon in Hades army, however, he bears a burdensome mandate: to join forces with the human entrusted to his protection. Unveiled by the ruler of the Underworld, the woman's true identity shatters Baal's resolve as he swears off love. Yet, as she threatens to unravel him entirely, he realizes his pledge may prove impossible to uphold.
When her sibling vanishes without a trace, Ranata becomes ensnared in the clutches of those who seek to offer her as a sacrificial offering to the depths of Hell. As a power-hungry demigod plunges the world into chaos, with her sister now in his grasp, Ranata unearths her own nonhuman origins and confronts the paramount test of her existence. Entwining herself with a demon defies all reason, but in her quest to rescue her sister, she is prepared to defy sanity itself.
Yet, surrendering to love him may be the epitome of madness.