Dark Deception - Rogues of Magic Series, #5

There's a killer on the loose in the Five Lands. When resistance members are slain in mysterious ritual sacrifices, Ann and Ceara are asked to help. When the only Council member sympathetic to the resistance goes missing, even Ann knows this is something she can't ignore. The ritual killings aren't the only problem on their plate. Ceara has been targeted, but by who-or what-she isn't sure. She suffers blackouts and a voice in her head accuses her of being the murderer.
Even she starts to believe it-until she realizes she's fighting for her life against the real killer. And it's a losing battle. There's a dark entity slowly invading her body. It's draining her more every day even as new victims are claimed by an enemy who is only growing in power. She can't keep this up forever. Time is running out. The killer must be found soon or Ceara will be added to the list of victims-and if the killer wins, no one in the Five Lands will be safe.