Its called "Persistent World" it pretty much makes mount and blade multiplayer like an mmo. You can gather resources, capture castles and the such, craft weapons. Its all really well done!
If you're going to download here's the link:
http://www.mbrepository.com/file.php?id=2636
If you're going to play make sure to join "NAPW_roleplay" or something around those lines. It's the largest and has a map based off of game of thrones.
my name on the server is jarl and the faction I'm part of is called hibernia so make sure to join them.
ALSO, one thing you might want to remember is that when you die, you lost ALL of your stuff. The only things you get to keep are your denars, even then you lose a few.
Now ride forth gentlemen! and help us conquer this land!