The Deer King, Vol. 2 (novel) — Nahoko Uehashi

Van's chase to rescue the kidnapped Yuna leads him to the Ahfal Oma, the People of the Fire Horse. They reveal that the sickness plaguing Aquafa is no random resurgence but a carefully orchestrated revenge plan, and they want the leader of the Lone Antlers to join. Van can't abide a destructive quest that threatens all of Aquafa, but he's still only one man. And how can a single man stop a disease -- especially one that already lives within him?