My thoughts.... So I just read all 50 books and I have theory. Eris is the last of the people that the hive became or a daughter perhaps. After we kill Oryx and Eris touches the sword and pledges to serve her queens promise, she wasn't talking to the Awoken queen, she was talking to her's, Taox. It was to the queen that fled the three sisters (Oryx, Xivu_Arath, Savath) that found the worm gods and became the hive. This is why Eris could hide among the Hive in Hellmouth and how she knows so much. She matches the description of their previous form, we think the three eyes were stolen from hive but she had them all along. The queen of the pre-hive race fled the sisters as they were gaining strength and they tracked her for millennia but never found her. A race of people found a cyro chamber with Taox in it and perhaps she lived past the 10 year life span of their people once removed from the fundenment. And I think the fundenment, pre-hive home world, was a gas giant. It speaks of seas of poison and raining metal and helium drinkers and heat/pressure, and molten liquid metal core, 52 moons, etc, sound like Jupiter or Saturn? Maybe that's why they chose Saturn's rings, somewhere deep inside, it reminded Oryx of home.