I'm reading the Empress lore tab and in chapter 5 it appears that Savathun used Umun to summon Xivu Arath (You are war, and I conjure you with war and blood). If you read the book of sorrows this is how Oryx and his sisters brought each other back from their ascendant planes. If I'm correct that means someone already beat her but didn't enter her plane to finish her off.
-
2 things 1. No Xivu wasn't dead, Umun just lured her with her summoning 2. Savathune and Xivu weren't dead in the books, they were just in their individual throne worlds. So when Oryx did an act of trickery and war in his sister's name they were freed from their throne
-
1 Reply<Hive have general summoning rituals, it's not just stuff to bring people back to the material realm.>