Soul of the Blade by Brenda J. Pierson
A legendary sword for a legendary assassin. At least, that’s what Aeo thought when he first took the enchanted Bok’Tarong for his own. But instead of fame and glory, his life was ended and his soul consumed by the sword. Now the Bok’Tarong has a new bearer who’s determined to set things right—mainly by evicting Aeo’s soul from the blade and continuing with her holy mission to destroy the mind-eating parasites the Bok’Tarong was made to kill. Yet the blade won’t work without a soul, and Aeo’s is the only one available.
Together, they could free humanity from these parasites forever. But doing so will require them to put their trust in dangerous new allies—and each other, which might be asking too much of them both.