Search - Possession (Daughters of the Moon, Bk 8)

From Amazon:

Serena hasn't been herself lately. She has been having sudden lapses of memory and acting like a completely different person-one she doesn't like very much. To add to her confusion, she knows that someone has been following her. It all started the night an elderly woman helped her. Now Serena is beginning to wonder, who was that woman? And what has she done to Serena?

Serena, a moon goddess with the power to read minds, is possessed by the spirit of a woman who, in the thirteenth century, betrayed the evil Atrox and has now entangled Serena and herself in a plot to overthrow him.