Every other day, there will be a portal spawning somewhere, All players will hear a sound notification (like Siege Time or Mining) and coordinates will be shown on chat window (similar to when a player house decays) indicating where the portal is. The portal will spawn monsters periodically, even more and more challenging. When X mobs are defeated, the final boss spawns. Once defeated, the boss will drop some unique loot.
If the players did not kill X (let's say 50, for instance) mobs in an hour, the boss will not spawn, the portal vanishes and the event ends unsucessfully.
Players will probably have to cooperate with each other to defeat the boss and its minions. Also, could be PvP after the fight over the shiny loot drops.