https://www.gamespot.com/articles/destiny-2-faction-rallies-when-they-start-how-they/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.