Hive and Fallen battle it out in the "Sky Watch" area, don't know how this happens though but seems pretty cool...
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I was riding around in my sparrow and I saw them battle I went away pulled out my sniper and just watched, it was just vandals dregs thralls and alcolyes. They all were pretty low health but I think the fallen had a little more soldiers but I didn't get to see who won because a group of guardians came and killed them all so in the end guardians always win