C72 Left Arm and Right Arm

Chu Hai's guess was not wrong. Yin Fire Cult and Evil Spirit Castle did indeed have their own secret arts, and it was also very evil.

However, evil was the common view of the world. Good and evil were all in one thought, and all depended on who cast the spell.

What were the other two boxes?

The two of them were very interested in this. Being so generous with their hands