Sunset Warrior Cycle Series / Beneath an opal Moon / Eric van Lustbader

Terrible death stalks the streets of teeming Sha’angh’sei, and bloody vengeance rides in its wake. No man — or woman — is safe.
Then ghastly terror strikes at Moichi, oath-brother to the Dai-San. At the Circus of Souls, Moichi discovers a prize, more enchanting, more deadly than he can imagine, and he’s pitched headlong into a mission of rescue and revenge, with the lovely, enigmatic Bujun warrior Chiisai at his side.
Together they search to the farthest reaches of the world, where the moon shines not in one color, but in many. There, beyond the limits of the human realm, waits a mad sorceress . . . and the Firemask, the fearsome key that will unlock every nightmare the world has ever known!