Once you wrap up the Curse of Osiris story, complete the first three Adventures on Mercury and then repeat one of them in its Heroic variant, you will have access to Lost Prophecy Verses at the Weapon Forge in the Lighthouse. Some of them will require you to gather Advanced Paradox Amplifiers in order to fulfill the prophecies and forge the weapons.
Brother Vance actually has this item, but will only give it to you if you can give him ten normal Paradox Amplifiers. In order to obtain these, you must complete Strikes, Crucible matches, or Heroic Adventures. The new resource has a chance to drop after completing them. If you’re extremely fortunate, you may even get a drop of an Advanced Paradox Amplifier straight away.
Advanced Paradox Amplifier
To get your ten normal Paradox Amplifiers, you can participate in Strikes, Crucible matches, and Heroic Adventures. Brother Vance says to do those activities on Mercury, but the Paradox Amplifiers appear to be dropping in the relevant activities regardless of location. That’s a good things since players have no direct control over where a Strike or Crucible match will take place.
Once you have your ten normal Paradox Amplifiers, return to Brother Vance on Mercury and trade them in for one Advanced Paradox Amplifier. It’ll cost you 15 Legendary Shards to do so, but you’re most likely drowning in those.
Once you have the required amount of Advanced Paradox Amplifiers, you can visit the Weapon Forge and forge your weapon with the fulfilled Lost Prophecy. Keep in mind that some of the prophecies will also require Concentrated Radiolarian Cultures.
For more information on Lost Prophecies, their various Verses, and what weapons can be forged, be sure to check out our Destiny 2 Guide Hub.