I just read the Grimoire Fallen Ghost Fragment. Wow, that was a cool little story. A Fallen and Guardian on each other's side. I really hope one day, that I see a DLC of Fallen teaming with Guardians!
Another awesome idea? A DLC letting us play as Fallen. Imagine that!
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The card really boils down to the old adage, "the enemy of my enemy is my friend" then takes a stab in the feels by adding "until we're done fighting our enemies and turn on each other because that is what our people do."