https://www.gamespot.com/articles/heres-destiny-2s-new-exotic-ghost-shell/1100-6455763/

Destiny 2's Curse of Osiris expansion launched a couple weeks ago, and players have already delved deep into it and uncovered a lot of cool Easter eggs and mysteries. One of those mysteries involved the Lost Prophecy quests, and now we know what happens when you complete all of them: you get a sweet Exotic Ghost shell.

Once you complete all of Curse of Osiris's campaign missions, all of Mercury's normal Adventures, and one Heroic Adventure on Mercury, Brother Vance will give you a Lost Prophecy: Verse 1 item. Collecting enough items (for Verse 1, that's a Concentrated Radiolarian Culture, composed of ten Radiolarian Cultures) allows you to complete the quest, get a Tablet, and redeem that Tablet for a Legendary weapon from the Forge. You can get more info on how to unlock the Forge here, and you can see all the weapons in the gallery below (ordered by Verse number).