Mount & Blade: Warband

Really great RPG where you can build up your own army and wage war. Also has some of the best overhaul mods on the workshop period. I'd recommend everyone to give it a try at least once.

One of the greatest, if not the greatest, medieval sandbox roleplaying game ever developed. The graphics and models are simple and not up to modern standards, but deliver just enough to immerse you in the world of Calradia. The visuals have their own simple charm to them - nothing spectacular, just enough to create that feeling of being in a living, rugged medieval world. The absolute freedom that this game grants the player is its greatest strength: the story is yours. Become a wandering sellsword, an honored lord, a ruthless bandit, a scheming merchant, or even carve out your own kingdom. A seemingly insignificant battle can turn into a long-running rivalry with another lord; that same lord can escape your capture and return to cause you trouble. The desire for honor, blood, or plunder can lead you to join a faction, command armies, and eventually conquer the world. Open battles and sieges can feel chaotic, brutal and surprisingly personal, particularly when you're fighting alongside the troops you've recruited and developed yourself. What makes Warband so special is that it doesn't try to constantly entertain you. It gives you a world, a set of mechanics, and the freedom to make your own entertainment. Years after its release, that design philosophy and immersion still makes it incredibly difficult to put down or recreate. It may look dated, and it certainly feels dated in places, but underneath those rough edges is one of the most memorable medieval sandbox experiences ever made. The modding community is also one of the best you could find. Overhaul mods, weapons, armor, you name it. Had enough of Calradia? Install a mod to play in Westeros during the War of the Five Kings or even Middle-Earth.

Pretty much the standard for a game you can put hundreds and thousands of hours into

Sir Jeremus my beloved.