Saiyuki.

Three years before our story began an unlikely party formed. The story of how Sanzo, Goku, Gojyo, and Hakkai met is revealed at last -- violence drew them close, but something far more powerful kept them together. Now with bonds forged in insecurity and despair, our party is trapped in an undergound youkai den with a malicious female demon. Her interest lies with Sanzo, but her strength lies with her poison ... and her ability to make it linger.