https://www.gamespot.com/articles/destiny-2-faction-rallies-update-when-they-start-h/1100-6453516/

Weapons tied to factions have begun to pop up in Destiny 2, but things will soon become much more involved. Bungie previously revealed that something called Faction Rallies would begin on September 26, but we had no idea of what those would actually entail. The developer has now provided details on how they'll work and what you can get by participating. [Update: Destiny 2's new patch is now online, which prepares for the game for Faction Rallies. As pictured below, you'll see markers on the map for the new NPCs, but they aren't actually in place yet.]

Faction Rallies are a special event that kicks off with the weekly reset on Tuesday, September 26, at 2 AM PT / 5 AM ET / 9 AM BST and runs until the same time on October 3. All three factions return from the original Destiny--Dead Orbit (represented by Arach Jalaal), Future War Cult (Lakshmi-2), and New Monarchy (Executor Hideo)--to task players with "gather[ing] supplies for their faction and destroy[ing] enemy resources." This is done by simply playing the game--you'll choose one of the three factions to align yourself with and then earn Faction Tokens by completing public events, Lost Sectors, Strikes, or the Raid, or by playing Crucible.