Saiyuki Vol. 2 by Kazuya Minekura

Genjyo Sanzo is a Buddhist priest in the city of Shangri-La, which is being ravaged by yokai spirits that have fallen out of balance with the natural order. His superiors send him on a journey far to the west to discover why this is happening and how to stop it. His companions are three yokai with human souls. But this is no day trip -- the four will encounter many discoveries and horrors on the way, and on the road, Sanzo will wonder... can he really trust his supernatural companions?