We're never going to fight Xivu Arath. We've been at this dumb storyline for 2 years now and I'm starting to think we'll never get to fight Xivu. After final shape, Xivu will still be a threat for some reason.
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Well typically the Hive gods were always the main threat of a whole expansion. Assuming Bungie has one more expansion in them, she may finally appear for that story. Or at least a final Raid.