Destiny Theories: Who is Eris Morn truly loyal to?
Eris Morn was once a guardian, a hunter. Eris was part of a six person fire team that challenged a god, and failed. Only Eris climbed from the pit where her comrade's perished, and she was forever changed. Haunted by her failures, and weighed down by the gruesome fate of her friends, she took it upon herself to pass on her knowledge to other guardians, so that they might finish what she started. It is apparent that her purpose is to defeat Crota's armies and see the dark prince killed. The Vanguards, leaders of the city's defenses, seem hesitant, even uneasy of her presence.
Evidence of an alterier motive #1
Once Crota fell, Eris finally found the peace she needed, the reassurance that her fallen brothers and sisters were avenged. The war however, was not over. The darkness still loomed on the horizon, and she refocused her attention to readying the Guardians for the next fight. Now, as it stands it may seem like Eris is loyal to the city. However, there is evidence leading towards a theory that she really serves another. In reality, Eris serves Mara Sov, Queen of the Awoken, ruler of the reef. First evidence in support of this theory is; Eris is a female Awoken, Throughout Destiny's story, encounters with the reef reveal that many of Eris's Bodyguards are female. Her champion and trusted adviser Petra Venj is also female. It appears the Queen favors female warriors to act has her champions.
2nd piece of evidence
The second piece of evidence can be found during the opening story cinematic for The Taken King expansion. In this cut scene, The Queen leads the Reefs fleet to halt Oryx's advance. Oryx is Crotas father and has come for revenge of his own. During this massive space battle, the Queen and Oryx exchange fire from their respective super weapons. As Oryx's Dreadnought unleashes it's weapon, the Awoken fleet is devastated, and it seems the Queen is lost. Mara Sov's apparent last words are "You and I know how this ends. We've known since you escaped from that...pit". As the cinematic comes to an end, you hear the Queens narration, and she say's "Guide them.... My hidden friend". Now, her reference to an escapee of the pit could be referring to the Guardians that killed Crota, but it would appear Mara Sov is addressing a singular being. Eris Morn alone crawled out of the pit. Further supporting this is the Queens reference to a "hidden friend" to guide the City's forces. Eris has indeed been guiding the Guardians to battle the hive. It does not seem she is doing this at the vanguards request, so she is most likely there on behalf of the Queen.
The last and arguable most valuable bit of evidence supporting the theory of Eris's true loyalty is shown in the final cinematic cut scene. Upon defeating Oryx., you triumphantly return to the city to receice your next assignment. After you have left, someone else comes to the spot of Oryx's demise. A Certain someone named Eris Morn. Eris approaches the husk of Oryx's sword. It splits open, revealing a dark purple crystal, floating over where Oryx fell. Eris then collects the crystal and the scene ends. As the scene plays out, Eris narrates saying "My Queen, you were right. The Guardian was the key". The only queen we know of Mara Sov, Queen of the Reef. Eris then comments she has accepted her fate and knows what she has to do, implying she still has a mission, a secret agenda that the Guardians don't know about. Whether or not this agenda is harmful or helpful to the traveler is unknown, but one thing is certain. Mara Sov and Eris need the Guardians help to achieve their goal, what ever it is.
Altar of Oryx
Destiny's full story has yet to be revealed, and it's lore has every bit of potential to expand. This is certain however: Mara Sov has an agent inside the City, and it is most likely Eris Morn. Eris is a female awoken, she survived the pit, she guides the Guardians against the Hive. Eris vows to her queen that she will not fail her task, regardless of the task.