Katainaka no Ossan, Kensei ni Naru

One of the most-watched Japanese light novels, which shows the interesting journey of a middle-aged man who turns into a sword saint. This series gained popularity among viewers and they are expecting it to be an anime, but it is a manga and Novel. 

The story takes a new turn when he gets extra power. He has a humble personality in external looks, but he has a special ability to fight intense battles. It is full of funny moments and action, which is mixed with fantasy elements.