Mount & Blade is a game that puts you in the saddle of a peasant in a strange land of warring medieval factions. You're free to train your skills and recruit soldiers, raise armies, fight for kings, destroy kingdoms, own land, trade goods, rinse the market, and, if you mess up, be taken prisoner, marched around the land, abandoned, and forced to start all over again.
There isn't a game quite like it for ambition and scope, yet it came from a tiny team.
Well, it's currently free on GOG and you'd be foolish to pass up the opportunity to pick it up.
If the Mount & Blade bug gets you, then check out the modding scene; it has been very active over the years, creating Lord of the Rings, Game of Thrones, and Star Wars conversions. Plus, tonnes of features and improvements, both mechanical and graphical.