So I think that Eris' rock could be a piece of Taox, or maybe all that is left of her. It could explain why it speaks to her and warned her of Oryxs arrival. Taox could even be the Queen that Eris was speaking to in the cutscene after you defeat Oryx, rather than Mara Sov. We know that Taox wants her spawn dead, and it seemed as if she was following the Traveler and helping those the Traveler blessed so they could kill her spawn. So it would make sense that she could have arrived in our system.