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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It's a random event, you get a little message before it happens saying "The enemy are moving against each other" then it starts to spawn waves of increasingly higher ranked enemies until you get the Hallowed Knights, Wizards, Servitors, Captains etc. Was one of my fave moments on the Beta (except maybe unleashing the hive on the moon, reminded me of the flood level in H1!)