The idea is that the evil forces have forced guthix and saradomin(and any other gods that don't approve of zombies everywhere) out of the game which is why zamorak and the gods of the dark have been in control, filling the world with skeletons and zombies and numerous other evil monsters. You can't attack other players in this minigame, only enemies. To allow teams to work together, there is multi-combat everywhere.
Every game has at least 40 players. Each player is transported to either varrock or lumbridge and then to the city. They all have to defend their home until they've been able to defeat all the zombies. they'll all appear in waves, each getting harder, the last wave is wave 5. once they have beat wave 5, they'll be transported by the spirits of dead warriors who perished fighting zombies to falador, where they'll meet with teams from the other cities. The first thing they must accomplish is to defeat the "hive" which is a massive, egg-like, gooey, egg-shaped shape that spews horrible guns at players. After that the boss zombie will emerge from it. It starts at level 400 and rises by 20 levels per player that remains on the field.