Basin-Hollow Emperor

The protagonist's father has a dream and thinks he is the true Son of Heaven, and then a Daoist priest from a ruined temple also makes a calculation with his fingers and says that he is the emperor destined to rule. So this small landowner, who had never even been to the prefectural city, ends up declaring himself emperor and founding a nation in a small mountain valley.

(Based on reader feedback, this book moves extremely fast, with a coup in three chapters and a full-scale national war every so often...)