My theory is the original Eris died in the Hellmouth and Savathun saw this as a golden opportunity to infiltrate The Last City. That's if she's capable of using a spell to disguise herself....which I'm sure The God of Trickery can. Also it fits my other theory that Savathun orchestrated Crotas and Oryxs death, to take the throne for herself. . Yet if that's the case. Why didn't Xivu Arath notice the empty throne or did she? . Another of my crazed theories is that Xivu is Umun'Arath in disguise. She'll be The Primus of all Legions and essentially top dog of a military focused race. Though that's only based off the "Arath" portion of the name and being Alpha of Alphas. Also a hole in that theory is why the disguise? She's the embodiment of War. Why wouldn't she just crush The Cabal Empire? Unless The Cabal are too strong for her. . Anyways...the general view is that Eris and Savathun or separate beings. Eris kicked rocks shortly before The Last City got nut tapped. She disappeared to do who knows what...the common theory (and most likely true one) is she went off the deal with more Hive BS. . Savathun on the other hand. She seized the void that Oryx left. Got his Taken powers either through taking his thrown....or by Quria BladeTransform. Though if Savathun noticed the empty throne.....did Xivu Arath. After all both went to great lengths to create some space between eachother, but primarily from Oryx. As one took all their minions into a wormhole (or was it a blackhole)........and the other to the edge of the galaxy. Savathun proclaimed they must become "different". Xivu Arath straight up told Oryx he's too big and they must part ways.

No.Eris is by no way part of the family. BoS only records 3 sisters, not 4 Eris litteraly had to get an acolite eye and insert it on her forehead to see in the dark below. Everything she had was taken by the hive, and it is even more possible for her to have a worm that to her being a sister. As to becoming the taken queen... She does have some knowledge, so it is possible.

<No, Savathun is the new Taken Queen... and me thinks that our ol' obscure buddy Quria is playing her game, but with a twist.>