First off, check out this theory if you have time.
(It’s not mine, all credit to OP. The rest of this page is my thoughts)
This theory states that Savathun is the heart of the Black Garden. Now, we know that the D1 base game was full of plot holes, so take that theory with a grain of salt. My idea is that Eris and Savathun are connected. Other people have said this, but I have more proof with D2’s coming.
In D1, the black garden (maybe Savathun) was defeated. Shortly after, Eris appeared. She helped you defeat Crota and Oryx. Then there was RoI, where she didn’t do much but was in the Tower still. Now we’re at D2, Eris hasn’t been seen since the Tower fell (as said by the Vanguard), and Savathun is referenced many times (Savathun’s Song, Lake of Shadows strikes) The timing of all this is perfect. Eris consistently disappears when Savathun appears. This is decent proof enough, but what about motive?
In the Books of Sorrow, Oryx, Savathun, and Xivu Arath like to kill each other. A lot. This lines up with Eris’s goal. There’s the motive right there. To make this even more interesting, Eris feels mental sensations about the Hive (fingers on my mind, etc). Crota has no mind related powers, and while Oryx does control the Taken, Eris is not Taken. This leads me to think that she is being manipulated, maybe unknowingly, by Savathun. Savathun is known for mental tricks and “fingers on my mind” seems right up her alley.
So yeah, there’s my first theory for the collective. Excited to hear what you guys think.