Enemies all have factions, we know this. But why do we really only care about the Fallen's Factions?
I bet you can't name all the Hive groups, or Vex programmings, or Cabal Legions. But I bet you could name all the Fallen houses. Why? Why are the Fallen so much more interesting?
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Another feeling I have is why they're so interesting is the fact they are our first introduction to a hostile alien species. They are constantly trying to salvage or occupy human golden age structures, they are our main obstacle to retaking what we lost.