The Juncture is in Venus's Ishtar Sink. To get there you need to travel to the area behind the Vault of Glass called the Citadel. In the Citadel, you should find the path to the Juncture. Inside, there are a bunch of lvl 22 enemies. Once you get past them there is this giant beam coming out of the sky landing inside of the room. This will probably be part of a content update, but what do you think will happen with it?
EDIT: It's not actually [b]in[/b] the vault, it's behind it.
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Possibly a new raid or, more likely, addition story content. I saw it for the first time today and it seems like a grav-lift up to the tower there. My guess is that (just like how the first DLC will be based on the Hive and the second will be the Fallen) it will be the new Vex DLC.