The Yin-Yang Fish and the Keeper of Flaming Mountains is a mandatory boss you must defeat to continue through the game. It's located in the Furnace Valley, near the Emerald Hall Shrine.

The Yin-Yang Fish can also "roar" and call orbs of light up out of the ground. These will damage you if you touch them, but serve no further purpose and can be avoided pretty easily.

Crimson-Silver Cart is a Yaoguai Chief boss fight that you'll go up against after defeating the Keeper of the Flaming Mountains and the Yin-Yang Fish in The Emerald Hall during Chapter 5 in Black ...