When Oryx invaded the System, Mara Sov met him in battle at Saturn's Rings. Her fleet challenged the Taken King in an effort to protect the Reef from further incursion. Her fleet was decisively defeated, and she was missing in action, but apparently Mara planned for this. The battle was simply a delaying action to aid the Guardians in defeating Oryx, as part of her secret alliance with Eris Morn. By keeping the Dreadnaught contained in the outer system, they bought Earth enough time to react and take the fight to Oryx.
[i]No body had been recovered after the battle[/i], but signs that her ship had not been totally obliterated were recovered by Taken champions, which were later acquired and passed on to Petra. After a search through the Dreadnaught, there was still no sign of the Queen, but Petra continues to hope that she survived.[i] Uldren, who survived the battle, also suspected that the Queen survived, as he could still sense her.[/i]
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I suspect Queen Mara Sov's return will be the main focus of a future DLC or maybe even an integral part of Destiny 2.
Some people believe that Mara Sov is the energy light sphere that helps guide guardians on the Dreadnaught missions (see pic)
This makes sense considering the Queen is confirmed to be alive and now that I think of it more, Mara Sov does say something around guide the guardians my mysterious friend. At the end of the CG intro for TTK
Seems legit or it could be Eris' weird powers that she learned from Toland the Shattered. Mine seems unlikely but I like counterarguments that open new views 😆