TLDR VERSION: eris is serving savathun. if you want to know more, continue reading. warning, this is super long, even though I didn't want it to be. it would've been even longer though if I explained all the lore behind it. anyway, the following wall of text is worth a read if you want to know what's really going on with the story of destiny. I don't explain all the lore, so it will definitely help for you to know some of the lore already before reading this. anyway, enjoy! eris isn't serving queen Mara sov like some people think (although they are probably working together, eris does not serve queen Mara sov) that much i know. ok, so if you read the grimoire you know oryx is one of three siblings who were originally named aurash, Xi ro, and sathona. I'm going to assume you already know some of the backstory and i'll try to make the REALLY long story as short as i can. they all used to be female but two of them turned male once they basically sold their souls to the worms (their gods) to gain power and immortality, but at a great cost and I'll explain this later. once they did that, they took new names. aurash became the male auryx (auryx later titles himself king and takes the name oryx, whom we all know and love). Xi ro becomes a male knight xivu arath, and sathona becomes the female hive mother/witch savathun. as part of their deal with the worms, they must continuously follow their natures and if they don't they will die. savathun is the smart and cunning one, oryx is the curious one, and xivu arath is the mighty one. through a long series of events, oryx becomes the leader of the three and is eventually the strongest. he does this by repeatedly killing his siblings and then bringing them back to life. his siblings also kill him and bring him back to life multiple times. this process makes them all stronger according to this law they call sword logic (too hard to explain here. read lore if you want to understand). anyway, back to the point. at the end of the taken king story, we kill oryx in the real world and then in his own dimension, which supposedly kills him off for good (he can still be brought back though through certain circumstances). anyway, at the end of the story, we see eris morn go up to the oryx's sword (the source of his power) and take its essence and tells us that the guardians were the key. she is speaking to her "queen". this led people to believe it was queen Mara sov but it's not. it's savathun! savathun is basically queen of the hive. each of the siblings has their own sect of hive and savathun killed oryx many times in the past through trickery and deception to make herself stronger. basically, this means that savathun somehow orchestrated everything that has been going on in destiny up til now in order to kill and overtake her brother oryx to gain his power because when one sibling kills another they indirectly gain more power. savathun somehow orchestrated crota's invasion and knew that guardians, as agents of the light, would send an attack squad (eris's fireteam) down into the pit to kill crota. savathun knew this and somehow, corrupted eris and turned eris into her servant, basically brainwashing her into being an inside agent of the hive. savathun knows that she is weaker than oryx and she knew that oryx was weaker than guardians. her ultimate goal is to destroy us but she knows she can't at her current level. so what was she to do? she deduced that she had to become more powerful in order to kill us but she could only gain power if she somehow killed off oryx in a way that coincides with her own nature of cunning and trickery as part of her pact with her worm god. basically, she used eris to orchestrate our raid on crota in order to lure oryx to us, knowing he would seek revenge. then savathun used eris to coordinate us to kill oryx so that she could obtain his power. that's why she takes that thing out of oryx's sword. it's the source of his power, according to sword logic and she had eris steal it in order to make her stronger in the hopes that it would be strong enough to take on us. savathun is the criminal mastermind behind almost everything that's been going in so far and this is blowing my mind after putting all the pieces together! let me know what you think below! edit#1: some commenters asked a great question: how is savathun stealing oryx's power if eris put the stone from oryx's sword into our exotic swords? there are two reasons why savathun would become stronger by getting rid of oryx: one reason is sword logic! it's complicated, but because eris tricked us into using oryx's power to kill things with our sword, we're indirectly feeding savathun more and more power. she tricked us, as is her nature which is enough to transfer the power of what we kill with our swords to her. this is why we get the exotic quest to turn our legendary sword into the exotic version. as the sword becomes stronger from killing things, it changes form. the other reason is that every time savathun follows her nature, the worm inside her becomes satisfied, which gives her more power directly proportional to how much more she satisfies her worm's hunger than she is used to. the catch to this is that the more powerful her worm makes her, the hungrier it becomes and the more cunning and trickery is required for her to remain alive or else her worm will start to feed on her and eventually kill her. there's a lot to this and it's hard to explain but read the lore in the grimoire and you'll start to understand how this works, I promise! edit#2: some brought up queen mara's statements about her "hidden friend" in the beginning conenatic. no doubt she is referring to eris in the tower. the awoken, as was already known are neither light or dark. they merely do what they must to survive. I believe mara and savathun were somehow working together for one reason or another. this would explain why mara sov is the only awoken to have incredible powers. maybe savathun lent her some power as part of her plan. there are lots of possibilities. and there are lots of gaps to fill with future storylines.