This is book 5 in The Vampire & Werewolf Chronicles.

When Sophie's old friend, Beth, shows up at her doorstep and needs a place to stay, she is hesitant to let her stay at the castle; especially with her new immortal roommates and her new dangerous, immortal lifestyle. More chaos breaks out while she's hunting for the necklace that is the key to open the portal to hell. Sophie also deals with the fact that Logan will be marrying her roommate to stop the curse leashed upon the land.