Like the Vex or Cabal invading Earth and you have to defend against their attack for x amount of time.
Edit: wow this thread is tending now? Amazing! Hopefully Bungie reads it!
English
#Destiny
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Like one huge battle that lasts for X amount of days, drop in drop out, periodic boss spawns that guarantee a 2-3 legendary Engrams, and a small chance at an Exotic engram. But the Engrams need to be special and only have exclusive items for the invasion.