Dark Flame: The Immortals

Dark Flame was written by Alyson Noel.  It is book 4 in The Immortal series.

From alysonnoel.com:

In this eagerly awaited continuation of the bestselling Immortals series, Ever struggles to help her best friend Haven transition into life as an immortal—trying to keep her from doing anything that puts them at risk, while attempting to gain control over her enemy Roman, so she can finally obtain the antidote that will allow her and Damen to be together. But when the spell she casts backfires, resulting in a strange, foreign pulse that binds her to Roman instead, Ever turns to Jude and dark magick, desperately attempting to break free of the curse, and ultimately risking everything she knows and loves—including her beloved Damen.

 All I really want to say is.... Roman, Roman Roman.  Talk about just loving to hate the "villain".  I hate to give anything away... so I'll just say that by the end of this book I loved Roman... and was glad Ever made me feel for him.