The Heaven-Mandated Yin-Yang Master

My grandfather was a Yin-Yang master of profound skill. He often said that in this world, humans have their own way and demons have theirs. All things must obey the rules, but some people still try to upset the balance. He devoted his life to keeping the realms in balance, yet his final instructions left me utterly baffled. He insisted that I dig up his grave on the third anniversary of his death...