The Darkest Whisper: Lords of the Underworld, Book 4 (Unabridged‪)‬

Bound by the demon of Doubt, Sabin unintentionally destroys even the most confident of lovers. So the immortal warrior spends his time on the battlefield instead of the bedroom, victory his only concern...until he meets Gwendolyn the Timid. One taste of the beautiful redhead, and he craves more.

Gwen, an immortal herself, always thought she'd fall for a kind human who wouldn't rouse her darker side. But when Sabin frees her from prison, battling their enemies for the claim to Pandora's box turns out to be nothing compared to the battle Sabin and Gwen will wage against love.