Shade is Eidolon' brother, i.e. a Seminus demon and consequently a male-slut. He cannot help it since he needs sex in order to survive. 
 On the other hand he is so lost in his little bubble where everything ends up in sex, that he does not really pays attention to people's feelings and the way he truly hurts them. 
 Runa used to be his lover. She also used to be human i.e she had no clue about demons, the demon world and especially Seminus demons that use sex for survival. 
 Runa met Shade while she was working. He was good-looking, sexy, pushy (they had sex almost immediately after they met) and irresistible. All Seminus demons are irresistible... 
 So when she found him in bed doing a paranormal threesome, she ran away and got bitten by a warg (werewolf). It was her lucky night I suppose. 
 So now she's a werewolf and she absolutely hates Shade for his betrayal, while she considers him responsible for what happened to her. 
 The start of the book finds them chained up together in a dungeon. Neither of them knows what they are doing there, but they will need to cooperate in order to escape and survive. 
 They will be forced into bonding in the Seminus way and Runa will show her teeth. She is not that weak, stupid human anymore and Shade has a hard time not to fall in love with her. 
 There is some BDSM because Shade is into it and he uses it to help women heal inner wounds. But nothing too much. And nothing too believable and serious to be honest... 
 I did not particularly like Runa in this book; however she grew on me after I read the next books in the series. 
 The cave where Shade and Runa are hiding, far away from everybody is also playing a vital role in the next books.