Full Invasion 2 is a co-op module for Mount & Blade: Warband where players come together to survive countless waves of invading enemies and bosses. It has been voted one of the most popular multiplayer modules within the community, and is still under active development to this day.
It's a really fun mod and it still has a small community playing. That being said, you can have fun with just two people or even solo. My only gripe is that some of the factions need work. The Imperium of Man is very disappointing, for instance. The LOTR, Japan, Rome, China, and a couple others are very, very fun to play as. The game also has a horde of socialists that you can defend against. Fighting EU stormtroopers and drunk Russian peasants who threw vodka bottles made my day.