https://www.gamespot.com/articles/destiny-2-curse-of-osiris-forge-guide-how-to-unloc/1100-6455416/

One of the key post-campaign activities for Destiny 2 players to check out after finishing the new Curse of Osiris DLC expansion's campaign are Lost Prophecies. Completing these reward you with unique new, Vex-style weapons, but you don't immediately gain access to any of this. Here's how to unlock Lost Prophecies.