The Yin Tiger is found in Zodiac Village, the secret hub area you can unlock by following Chen Loong's quest. To summarise, follow these steps: Defeat Chen Loong. Talk with Xu Dog at the Cellar in Chapter 2. Receive the Special-Made Bone-Strengthening Pellet from Xu Dog. Return the Pellet to Chen Loong to unlock Zodiac Village. Once you're inside Zodiac Village, head down the left-hand path and you'll soon reach the Yin Tiger, smithing away as he always does. Speak to him, and you'll have the option to Challenge him. Beware - the Yin Tiger is a very deadly foe, probably the most dangerous boss in Chapter 3. His massive sword strikes deal colossal damage, and his long attack patterns are tricky to master. He is resistant to Immobilize, and A Pluck Of Many doesn't do terribly well against him. My advice is to unlock Spell Binder by defeating the Green-Capped Martialist, and then use it to exchange your Mana for attack damage since your Spells aren't of great use here. Stay patient, learn his moves, and stay aggressive. This is the most Souls-y boss so far. Once you defeat Yin Tiger, you'll gain his excellent Ebon Flow Transformation Spell, as well as the ability to unlock a new curio slot from him once you speak to him again!