Forest Mage by Robin Hobb

Plague has ravaged the prestigious King's Cavalla of Gernia, decimating the ranks of both cadets and instructors. Yet Nevare Burvelle has made an astonishingly robust recovery, defeating his sworn nemesis while in the throes of the disease and freeing himself—he believes—from the Speck magic that infected him. And now he is journeying home to Widevale, anticipating a tender reunion with his beautiful fiancée, Carsina, and a bright future as a commissioned officer.

But there is no haven in the bosom of his kinfolk, for his nights are haunted by grim visions of treachery—and his days are tormented by a strange side-effect of the plague that shames his family and repulses the lady of his heart. And as the still-potent magic in his blood roars to life, Nevare realizes a terrible truth: that the enemy who seeks to destroy everything he loves dwells perhaps not without but within him.