I had originally posted that this was related to Xur, but i was wrong. The "Emissary from Beyond" is actually connected to a PvP game mode called the "Trials of the Nine". You must have completed the story before you can get this milestone. Once you have the milestone, you must reach level 260 and then you can progress with the milestone and get the trophy for meeting the emissary. Check out the description in the trophy guide here.