When someone beats the Trials of Osiris with 9 wins and 0 losses a new level on Mercury will unlock for them. It's a new secret social space. I'm wondering if I was in a fireteam with someone who had unlocked it would they be able to take me there?
For example, for the weekly nightfall, sometimes if my alt hadn't finished the Eris Quests and the nightfall was in a level that came from the Crota expansion, then I wouldn't have been able to select it but my friend could take me in if it was unlocked for him because he had finished the Eris quests. Maybe this is also the case with Mercury?
Here's a video of the place I'm talking about: