Basically, it's a game like L4D2, but the only way to survive there is to kill literally everything. Also, the first one was hard as hell, the boss (called the patriarch) spawns with a minigun, a rocket launcher, can heal three times, spawns a few monsters, and become invisible when he needs to hide (not OP at all, uh). And you fight it if you manage to survive the whole 4/7/10 waves depending of the lenght of the game. It's a shooterfest, but you also get murdered brutally if your team and you don't play well.