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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Xivu is real. She has talked to us. Savathun is going to help us to get into the traveler so we can go after the witness after her sister Xivu is dead.
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To be fair, it took us a while before we can fight Savathun when she was hinted during The Red War Campaign.
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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.