The Emperor of the Mountain Valley

The protagonist's father had a dream and thought he was the Son of Heaven. Then a Taoist from a run-down temple read his fortune and said he was destined to be emperor. And so, this little landowner, who had never even been to the prefectural city, declared himself emperor and founded a country in a remote mountain valley.

(According to reader feedback, this book moves at lightning speed: he becomes emperor in just three chapters, and before you know it, the whole nation is at war…)