+1 on this, M&B with the difficulty settings cranked is the best you can get atm.
+2, and even the highest difficulty singleplayer is nothing compared to multiplayer. Beware, though, it has an extremely steep learning curve.
While Mount and Blade is one of my favorite games of all time, its sword fighting is pretty different to the simulation in Die by the Sword. DbtS didn't really have pre-programmed motions like M&B, it instead attempted to map 2D mouse position to 3D hand position using a physical model of the player's skeleton (which generally resulted in flailing madly with your arm contorted into bonkers positions for the first week of play).
Also there is Dark Messiah of Might and Magic in the First Person Sworder(?) range.