I think you already know whats about to happen! 'All Guardians stand alert! Report to your class master now! There has been a breach in the wall!'
9 guardians (max. 3 per fireteam) vs. An army (5000-10000 units) of attacking Fallen/Hive. With the new subclasses coming and awesome new gear, 9 guardians will defend the Tower from invading Fallen/Hive.
This is an event like Iron Banner. It could happen anytime, it has a time duration 3 days and in those 3 days it could happen just once or multiple times. The intire 3 days could even happen twice a week or just once a month.
The enemies will come in waves of 100-1000 enemies each wave has a timer, so when you are too late the next legion will come!
The way the loot system works is that you will get (per wave) a loot boon/buff where engrams drop faster and sometimes there will be an exotic drop!
Feel free to give constructive criticism.
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Your math is a little off... 16 Guardians broken down into 6 man teams leaves 4 of them squadless. Better rethink this Mr. Code Red, because the only thing alarming here is that earth's last bastion of humanity will be defended by someone who can't even math. Scary..