The plot mainly about disease that infected one breed, then slowly it started to create chaos among one species but then the disease mutated and now it spread to another species, it might spread to all of species in underworld realm and it all started with Sin.
Sin is a succubus and she could heal but she could kill, too. That makes her a complete awesome package of sexy yet deadly. And there's Luc. He played a very good love interest to Sin but regardless of what he did to Sin to make Sin a 'better' person, he played a good role to help Sin to cure the disease.
Adventures were there but I didn't exactly enjoy them because it was all revolved in one thing: The Incurable Disease. The 4-star was given because Sin was awesome for being kickass, the romance was sexy, and the moment I read a man in armor on horseback, I instantly knew that must be the one of the Riders of Apocalypse which indeed, yes.
I'd be most likely to continue with this series if there is Book 6 on-making. It was amazing experiences to get to know all Seminus demons including all other species of the underworld as well.