Her Sinful Angel by Felicity Heaton

Cast out of Heaven and now the king of Hell, Lucifer is a powerful fallen angel warrior with a heart as cold as ice and soul as black as the bottomless pit. For millennia, he has ruled his realm with an iron fist as he plots the demise of his ancient enemies.

When one of those enemies dumps an unconscious mortal female in the courtyard of his fortress and leaves her there, Lucifer finds himself entranced by the beguiling beauty and tempted beyond all reason.

But is the enchanting Nina an innocent pawn in the eternal game or part of a plot against him?